diff --git a/stix2/properties.py b/stix2/properties.py
index 80c9e0a..735bc79 100644
--- a/stix2/properties.py
+++ b/stix2/properties.py
@@ -8,10 +8,7 @@ import re
 import uuid
 
 from .base import _STIXBase
-from .exceptions import (
-    CustomContentError, DictionaryKeyError, MissingPropertiesError,
-    MutuallyExclusivePropertiesError, STIXError,
-)
+from .exceptions import CustomContentError, DictionaryKeyError, STIXError
 from .parsing import parse, parse_observable
 from .registry import STIX2_OBJ_MAPS
 from .utils import _get_dict, get_class_hierarchy_names, parse_into_datetime
@@ -503,7 +500,7 @@ class ReferenceProperty(Property):
                 (valid_types is None and invalid_types is None):
             raise ValueError(
                 "Exactly one of 'valid_types' and 'invalid_types' must be "
-                "given"
+                "given",
             )
 
         if valid_types and not isinstance(valid_types, list):
@@ -517,21 +514,6 @@ class ReferenceProperty(Property):
         self.types = set(valid_types or invalid_types)
         self.auth_type = self._WHITELIST if valid_types else self._BLACKLIST
 
-        # Handling both generic and non-generic types in the same constraint
-        # complicates life... let's keep it simple unless we really need the
-        # complexity.
-        self.generic_constraint = any(
-            t in self._OBJECT_CATEGORIES for t in self.types
-        )
-
-        if self.generic_constraint and any(
-            t not in self._OBJECT_CATEGORIES for t in self.types
-        ):
-            raise ValueError(
-                "Generic type categories and specific types may not both be "
-                "given"
-            )
-
         super(ReferenceProperty, self).__init__(**kwargs)
 
     def clean(self, value, allow_custom):
@@ -543,10 +525,19 @@ class ReferenceProperty(Property):
 
         obj_type = value[:value.index('--')]
 
-        types = self.types
+        # Only comes into play when inverting a hybrid whitelist.
+        # E.g. if the possible generic categories are A, B, C, then the
+        # inversion of whitelist constraint "A or x" (where x is a specific
+        # type) is something like "[not (B or C)] or x".  In other words, we
+        # invert the generic categories to produce a blacklist, but leave the
+        # specific categories alone; they essentially become exceptions to our
+        # blacklist.
+        blacklist_exceptions = set()
+
+        generics = self.types & self._OBJECT_CATEGORIES
+        specifics = self.types - generics
         auth_type = self.auth_type
-        if allow_custom and auth_type == self._WHITELIST and \
-                self.generic_constraint:
+        if allow_custom and auth_type == self._WHITELIST and generics:
             # If allowing customization and using a whitelist, and if generic
             # "category" types were given, we need to allow custom object types
             # of those categories.  Unless registered, it's impossible to know
@@ -554,60 +545,40 @@ class ReferenceProperty(Property):
             # permissive approach and allow any type which is not known to be
             # in the wrong category.  I.e. flip the whitelist set to a
             # blacklist of a complementary set.
-            types = self._OBJECT_CATEGORIES - types
             auth_type = self._BLACKLIST
+            generics = self._OBJECT_CATEGORIES - generics
+            blacklist_exceptions, specifics = specifics, blacklist_exceptions
 
         if auth_type == self._WHITELIST:
-            # allow_custom is not applicable to "whitelist" style object type
-            # constraints, so we ignore it.
-            has_custom = False
-
-            if self.generic_constraint:
-                type_ok = _type_in_generic_set(
-                    obj_type, types, self.spec_version
-                )
-            else:
-                type_ok = obj_type in types
-
-            if not type_ok:
-                raise ValueError(
-                    "The type-specifying prefix '%s' for this property is not "
-                    "valid" % obj_type
-                )
+            type_ok = _type_in_generic_set(
+                obj_type, generics, self.spec_version
+            ) or obj_type in specifics
 
         else:
-            # A type "blacklist" was used to describe legal object types.  We
-            # must enforce the type blacklist regardless of allow_custom.
-            if self.generic_constraint:
-                type_ok = not _type_in_generic_set(
-                    obj_type, types, self.spec_version
+            type_ok = (
+                not _type_in_generic_set(
+                    obj_type, generics, self.spec_version,
                 )
-            else:
-                type_ok = obj_type not in types
+                and obj_type not in specifics
+            ) or obj_type in blacklist_exceptions
 
-            if not type_ok:
-                raise ValueError(
-                    "The type-specifying prefix '%s' for this property is not "
-                    "valid" % obj_type
-                )
+        if not type_ok:
+            raise ValueError(
+                "The type-specifying prefix '%s' for this property is not "
+                "valid" % obj_type,
+            )
 
-            # allow_custom=True only allows references to custom objects which
-            # are not otherwise blacklisted.  We need to figure out whether
-            # the referenced object is custom or not.  No good way to do that
-            # at present... just check if unregistered and for the "x-" type
-            # prefix, for now?
-            type_maps = STIX2_OBJ_MAPS[self.spec_version]
+        # We need to figure out whether the referenced object is custom or
+        # not.  No good way to do that at present... just check if
+        # unregistered and for the "x-" type prefix, for now?
+        has_custom = not _type_in_generic_set(
+            obj_type, self._OBJECT_CATEGORIES, self.spec_version,
+        ) or obj_type.startswith("x-")
 
-            has_custom = obj_type not in type_maps["objects"] \
-                and obj_type not in type_maps["observables"] \
-                and obj_type not in ["relationship", "sighting"]
-
-            has_custom = has_custom or obj_type.startswith("x-")
-
-            if not allow_custom and has_custom:
-                raise CustomContentError(
-                    "reference to custom object type: " + obj_type,
-                )
+        if not allow_custom and has_custom:
+            raise CustomContentError(
+                "reference to custom object type: " + obj_type,
+            )
 
         return value, has_custom
 
@@ -624,7 +595,9 @@ def _type_in_generic_set(type_, type_set, spec_version):
     result = False
     for type_id in type_set:
         if type_id == "SDO":
-            result = type_ in type_maps["objects"]
+            result = type_ in type_maps["objects"] and type_ not in [
+                "relationship", "sighting",
+            ]  # sigh
         elif type_id == "SCO":
             result = type_ in type_maps["observables"]
         elif type_id == "SRO":
diff --git a/stix2/test/v20/test_properties.py b/stix2/test/v20/test_properties.py
index af1ffec..4dbc437 100644
--- a/stix2/test/v20/test_properties.py
+++ b/stix2/test/v20/test_properties.py
@@ -84,19 +84,21 @@ def test_id_property_default():
     assert ID_PROP.clean(default) == (default, False)
 
 
-def test_reference_property():
-    ref_prop = ReferenceProperty(valid_types="my-type", spec_version="2.0")
+def test_reference_property_whitelist_standard_type():
+    ref_prop = ReferenceProperty(valid_types="identity", spec_version="2.0")
+    result = ref_prop.clean(
+        "identity--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False,
+    )
+    assert result == ("identity--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
 
-    assert ref_prop.clean("my-type--00000000-0000-4000-8000-000000000000", False)
     with pytest.raises(ValueError):
-        ref_prop.clean("foo", False)
+        ref_prop.clean("foo--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
 
-    # This is not a valid V4 UUID
     with pytest.raises(ValueError):
-        ref_prop.clean("my-type--00000000-0000-0000-0000-000000000000", False)
+        ref_prop.clean("foo--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
 
 
-def test_reference_property_whitelist_type():
+def test_reference_property_whitelist_custom_type():
     ref_prop = ReferenceProperty(valid_types="my-type", spec_version="2.0")
 
     with pytest.raises(ValueError):
@@ -105,16 +107,18 @@ def test_reference_property_whitelist_type():
     with pytest.raises(ValueError):
         ref_prop.clean("not-my-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
 
-    result = ref_prop.clean("my-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
-    assert result == ("my-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+    with pytest.raises(CustomContentError):
+        # This is the whitelisted type, but it's still custom, and
+        # customization is disallowed here.
+        ref_prop.clean("my-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
 
-    result = ref_prop.clean("my-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
-    assert result == ("my-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+    result = ref_prop.clean("my-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
+    assert result == ("my-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
 
 
 def test_reference_property_whitelist_generic_type():
     ref_prop = ReferenceProperty(
-        valid_types=["SCO", "SRO"], spec_version="2.0"
+        valid_types=["SCO", "SRO"], spec_version="2.0",
     )
 
     result = ref_prop.clean("file--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
@@ -124,12 +128,19 @@ def test_reference_property_whitelist_generic_type():
     assert result == ("file--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
 
     result = ref_prop.clean(
-        "sighting--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False
+        "sighting--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False,
     )
     assert result == ("sighting--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
 
     result = ref_prop.clean(
-        "some-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True
+        "sighting--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True,
+    )
+    assert result == ("sighting--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+
+    # The prop assumes some-type is a custom type of one of the generic
+    # type categories.
+    result = ref_prop.clean(
+        "some-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True,
     )
     assert result == ("some-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
 
@@ -143,7 +154,7 @@ def test_reference_property_whitelist_generic_type():
         ref_prop.clean("identity--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
 
 
-def test_reference_property_blacklist_type():
+def test_reference_property_blacklist_standard_type():
     ref_prop = ReferenceProperty(invalid_types="identity", spec_version="2.0")
     result = ref_prop.clean(
         "malware--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False,
@@ -155,6 +166,11 @@ def test_reference_property_blacklist_type():
     )
     assert result == ("malware--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
 
+    with pytest.raises(CustomContentError):
+        ref_prop.clean(
+            "some-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False,
+        )
+
     result = ref_prop.clean(
         "some-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True,
     )
@@ -170,15 +186,10 @@ def test_reference_property_blacklist_type():
             "identity--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True,
         )
 
-    with pytest.raises(CustomContentError):
-        ref_prop.clean(
-            "some-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False,
-        )
-
 
 def test_reference_property_blacklist_generic_type():
     ref_prop = ReferenceProperty(
-        invalid_types=["SDO", "SRO"], spec_version="2.0"
+        invalid_types=["SDO", "SRO"], spec_version="2.0",
     )
 
     result = ref_prop.clean(
@@ -191,16 +202,16 @@ def test_reference_property_blacklist_generic_type():
     )
     assert result == ("file--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
 
+    with pytest.raises(CustomContentError):
+        ref_prop.clean(
+            "some-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False,
+        )
+
     result = ref_prop.clean(
         "some-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True,
     )
     assert result == ("some-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
 
-    with pytest.raises(ValueError):
-        ref_prop.clean(
-            "identity--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True,
-        )
-
     with pytest.raises(ValueError):
         ref_prop.clean(
             "identity--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False,
@@ -208,7 +219,7 @@ def test_reference_property_blacklist_generic_type():
 
     with pytest.raises(ValueError):
         ref_prop.clean(
-            "relationship--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True,
+            "identity--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True,
         )
 
     with pytest.raises(ValueError):
@@ -216,18 +227,57 @@ def test_reference_property_blacklist_generic_type():
             "relationship--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False,
         )
 
-    with pytest.raises(CustomContentError):
+    with pytest.raises(ValueError):
         ref_prop.clean(
-            "some-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False,
+            "relationship--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True,
         )
 
 
-def test_reference_property_hybrid_constraint_type():
-    with pytest.raises(ValueError):
-        ReferenceProperty(valid_types=["a", "SCO"], spec_version="2.0")
+def test_reference_property_whitelist_hybrid_type():
+    p = ReferenceProperty(valid_types=["a", "SCO"], spec_version="2.0")
+
+    result = p.clean("file--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+    assert result == ("file--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+
+    result = p.clean("file--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
+    assert result == ("file--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+
+    with pytest.raises(CustomContentError):
+        # although whitelisted, "a" is a custom type
+        p.clean("a--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+
+    result = p.clean("a--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
+    assert result == ("a--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
 
     with pytest.raises(ValueError):
-        ReferenceProperty(invalid_types=["a", "SCO"], spec_version="2.0")
+        p.clean("b--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+
+    # should just assume "b" is a custom SCO type.
+    result = p.clean("b--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
+    assert result == ("b--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
+
+
+def test_reference_property_blacklist_hybrid_type():
+    p = ReferenceProperty(invalid_types=["a", "SCO"], spec_version="2.0")
+
+    with pytest.raises(ValueError):
+        p.clean("file--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+
+    with pytest.raises(ValueError):
+        p.clean("file--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
+
+    with pytest.raises(ValueError):
+        p.clean("a--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+
+    with pytest.raises(ValueError):
+        p.clean("a--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
+
+    with pytest.raises(CustomContentError):
+        p.clean("b--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+
+    # should just assume "b" is a custom type which is not an SCO
+    result = p.clean("b--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
+    assert result == ("b--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
 
 
 def test_reference_property_impossible_constraint():
diff --git a/stix2/test/v20/test_report.py b/stix2/test/v20/test_report.py
index 2de9363..29d181c 100644
--- a/stix2/test/v20/test_report.py
+++ b/stix2/test/v20/test_report.py
@@ -144,8 +144,8 @@ def test_report_on_custom():
             published="2016-01-20T17:00:00Z",
             object_refs=[
                 "indicator--a740531e-63ff-4e49-a9e1-a0a3eed0e3e7",
-                "some-type--2672975a-ce1e-4473-a1c6-0d79868930c7"
-            ]
+                "some-type--2672975a-ce1e-4473-a1c6-0d79868930c7",
+            ],
         )
 
     report = stix2.v20.Report(
@@ -154,10 +154,10 @@ def test_report_on_custom():
         published="2016-01-20T17:00:00Z",
         object_refs=[
             "indicator--a740531e-63ff-4e49-a9e1-a0a3eed0e3e7",
-            "some-type--2672975a-ce1e-4473-a1c6-0d79868930c7"
+            "some-type--2672975a-ce1e-4473-a1c6-0d79868930c7",
         ],
         allow_custom=True,
     )
 
     assert "some-type--2672975a-ce1e-4473-a1c6-0d79868930c7" \
-       in report.object_refs
+        in report.object_refs
diff --git a/stix2/test/v21/test_malware_analysis.py b/stix2/test/v21/test_malware_analysis.py
index e5ad628..ed8cce5 100644
--- a/stix2/test/v21/test_malware_analysis.py
+++ b/stix2/test/v21/test_malware_analysis.py
@@ -91,7 +91,7 @@ def test_malware_analysis_custom_sco_refs():
         product="super scanner",
         analysis_sco_refs=[
             "file--6e8c78cf-4bcc-4729-9265-86a97bfc91ba",
-            "some-object--f6bfc147-e844-4578-ae01-847979890239"
+            "some-object--f6bfc147-e844-4578-ae01-847979890239",
         ],
         allow_custom=True,
     )
@@ -105,8 +105,8 @@ def test_malware_analysis_custom_sco_refs():
             product="super scanner",
             analysis_sco_refs=[
                 "file--6e8c78cf-4bcc-4729-9265-86a97bfc91ba",
-                "some-object--f6bfc147-e844-4578-ae01-847979890239"
-            ]
+                "some-object--f6bfc147-e844-4578-ae01-847979890239",
+            ],
         )
 
     with pytest.raises(stix2.exceptions.InvalidValueError):
@@ -115,7 +115,7 @@ def test_malware_analysis_custom_sco_refs():
             analysis_sco_refs=[
                 "file--6e8c78cf-4bcc-4729-9265-86a97bfc91ba",
                 # standard object type; wrong category (not SCO)
-                "identity--56977a19-49ef-49d7-b259-f733fa4b7bbc"
+                "identity--56977a19-49ef-49d7-b259-f733fa4b7bbc",
             ],
             allow_custom=True,
         )
diff --git a/stix2/test/v21/test_properties.py b/stix2/test/v21/test_properties.py
index aa3624b..413aa0d 100644
--- a/stix2/test/v21/test_properties.py
+++ b/stix2/test/v21/test_properties.py
@@ -8,7 +8,7 @@ from stix2.exceptions import (
 from stix2.properties import (
     DictionaryProperty, EmbeddedObjectProperty, ExtensionsProperty,
     HashesProperty, IDProperty, ListProperty, ObservableProperty,
-    ReferenceProperty, STIXObjectProperty, StringProperty, TypeProperty,
+    ReferenceProperty, STIXObjectProperty, StringProperty,
 )
 from stix2.v21.common import MarkingProperty
 
@@ -87,19 +87,21 @@ def test_id_property_default():
     assert ID_PROP.clean(default) == (default, False)
 
 
-def test_reference_property():
-    ref_prop = ReferenceProperty(valid_types="my-type", spec_version="2.1")
+def test_reference_property_whitelist_standard_type():
+    ref_prop = ReferenceProperty(valid_types="identity", spec_version="2.1")
+    result = ref_prop.clean(
+        "identity--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False,
+    )
+    assert result == ("identity--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
 
-    assert ref_prop.clean("my-type--00000000-0000-4000-8000-000000000000", False)
     with pytest.raises(ValueError):
-        ref_prop.clean("foo", False)
+        ref_prop.clean("foo--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
 
-    # This is not a valid RFC 4122 UUID
     with pytest.raises(ValueError):
-        ref_prop.clean("my-type--00000000-0000-0000-0000-000000000000", False)
+        ref_prop.clean("foo--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
 
 
-def test_reference_property_whitelist_type():
+def test_reference_property_whitelist_custom_type():
     ref_prop = ReferenceProperty(valid_types="my-type", spec_version="2.1")
 
     with pytest.raises(ValueError):
@@ -108,16 +110,18 @@ def test_reference_property_whitelist_type():
     with pytest.raises(ValueError):
         ref_prop.clean("not-my-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
 
-    result = ref_prop.clean("my-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
-    assert result == ("my-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+    with pytest.raises(CustomContentError):
+        # This is the whitelisted type, but it's still custom, and
+        # customization is disallowed here.
+        ref_prop.clean("my-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
 
-    result = ref_prop.clean("my-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
-    assert result == ("my-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+    result = ref_prop.clean("my-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
+    assert result == ("my-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
 
 
 def test_reference_property_whitelist_generic_type():
     ref_prop = ReferenceProperty(
-        valid_types=["SCO", "SRO"], spec_version="2.1"
+        valid_types=["SCO", "SRO"], spec_version="2.1",
     )
 
     result = ref_prop.clean("file--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
@@ -127,12 +131,19 @@ def test_reference_property_whitelist_generic_type():
     assert result == ("file--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
 
     result = ref_prop.clean(
-        "sighting--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False
+        "sighting--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False,
     )
     assert result == ("sighting--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
 
     result = ref_prop.clean(
-        "some-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True
+        "sighting--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True,
+    )
+    assert result == ("sighting--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+
+    # The prop assumes some-type is a custom type of one of the generic
+    # type categories.
+    result = ref_prop.clean(
+        "some-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True,
     )
     assert result == ("some-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
 
@@ -146,7 +157,7 @@ def test_reference_property_whitelist_generic_type():
         ref_prop.clean("identity--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
 
 
-def test_reference_property_blacklist_type():
+def test_reference_property_blacklist_standard_type():
     ref_prop = ReferenceProperty(invalid_types="identity", spec_version="2.1")
     result = ref_prop.clean(
         "location--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False,
@@ -158,6 +169,11 @@ def test_reference_property_blacklist_type():
     )
     assert result == ("location--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
 
+    with pytest.raises(CustomContentError):
+        ref_prop.clean(
+            "some-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False,
+        )
+
     result = ref_prop.clean(
         "some-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True,
     )
@@ -173,15 +189,10 @@ def test_reference_property_blacklist_type():
             "identity--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True,
         )
 
-    with pytest.raises(CustomContentError):
-        ref_prop.clean(
-            "some-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False,
-        )
-
 
 def test_reference_property_blacklist_generic_type():
     ref_prop = ReferenceProperty(
-        invalid_types=["SDO", "SRO"], spec_version="2.1"
+        invalid_types=["SDO", "SRO"], spec_version="2.1",
     )
 
     result = ref_prop.clean(
@@ -194,16 +205,16 @@ def test_reference_property_blacklist_generic_type():
     )
     assert result == ("file--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
 
+    with pytest.raises(CustomContentError):
+        ref_prop.clean(
+            "some-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False,
+        )
+
     result = ref_prop.clean(
         "some-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True,
     )
     assert result == ("some-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
 
-    with pytest.raises(ValueError):
-        ref_prop.clean(
-            "identity--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True,
-        )
-
     with pytest.raises(ValueError):
         ref_prop.clean(
             "identity--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False,
@@ -211,7 +222,7 @@ def test_reference_property_blacklist_generic_type():
 
     with pytest.raises(ValueError):
         ref_prop.clean(
-            "relationship--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True,
+            "identity--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True,
         )
 
     with pytest.raises(ValueError):
@@ -219,18 +230,57 @@ def test_reference_property_blacklist_generic_type():
             "relationship--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False,
         )
 
-    with pytest.raises(CustomContentError):
+    with pytest.raises(ValueError):
         ref_prop.clean(
-            "some-type--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False,
+            "relationship--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True,
         )
 
 
-def test_reference_property_hybrid_constraint_type():
-    with pytest.raises(ValueError):
-        ReferenceProperty(valid_types=["a", "SCO"], spec_version="2.1")
+def test_reference_property_whitelist_hybrid_type():
+    p = ReferenceProperty(valid_types=["a", "SCO"], spec_version="2.1")
+
+    result = p.clean("file--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+    assert result == ("file--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+
+    result = p.clean("file--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
+    assert result == ("file--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+
+    with pytest.raises(CustomContentError):
+        # although whitelisted, "a" is a custom type
+        p.clean("a--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+
+    result = p.clean("a--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
+    assert result == ("a--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
 
     with pytest.raises(ValueError):
-        ReferenceProperty(invalid_types=["a", "SCO"], spec_version="2.1")
+        p.clean("b--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+
+    # should just assume "b" is a custom SCO type.
+    result = p.clean("b--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
+    assert result == ("b--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
+
+
+def test_reference_property_blacklist_hybrid_type():
+    p = ReferenceProperty(invalid_types=["a", "SCO"], spec_version="2.1")
+
+    with pytest.raises(ValueError):
+        p.clean("file--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+
+    with pytest.raises(ValueError):
+        p.clean("file--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
+
+    with pytest.raises(ValueError):
+        p.clean("a--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+
+    with pytest.raises(ValueError):
+        p.clean("a--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
+
+    with pytest.raises(CustomContentError):
+        p.clean("b--8a8e8758-f92c-4058-ba38-f061cd42a0cf", False)
+
+    # should just assume "b" is a custom type which is not an SCO
+    result = p.clean("b--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
+    assert result == ("b--8a8e8758-f92c-4058-ba38-f061cd42a0cf", True)
 
 
 def test_reference_property_impossible_constraint():
diff --git a/stix2/test/v21/test_report.py b/stix2/test/v21/test_report.py
index 69af096..1dc55be 100644
--- a/stix2/test/v21/test_report.py
+++ b/stix2/test/v21/test_report.py
@@ -4,12 +4,12 @@ import pytest
 import pytz
 
 import stix2
+from stix2.exceptions import InvalidValueError
 
 from .constants import (
     CAMPAIGN_ID, IDENTITY_ID, INDICATOR_ID, INDICATOR_KWARGS, RELATIONSHIP_ID,
     REPORT_ID,
 )
-from stix2.exceptions import InvalidValueError
 
 EXPECTED = """{
     "type": "report",
@@ -144,8 +144,8 @@ def test_report_on_custom():
             published="2016-01-20T17:00:00Z",
             object_refs=[
                 "indicator--a740531e-63ff-4e49-a9e1-a0a3eed0e3e7",
-                "some-type--2672975a-ce1e-4473-a1c6-0d79868930c7"
-            ]
+                "some-type--2672975a-ce1e-4473-a1c6-0d79868930c7",
+            ],
         )
 
     report = stix2.v21.Report(
@@ -153,10 +153,10 @@ def test_report_on_custom():
         published="2016-01-20T17:00:00Z",
         object_refs=[
             "indicator--a740531e-63ff-4e49-a9e1-a0a3eed0e3e7",
-            "some-type--2672975a-ce1e-4473-a1c6-0d79868930c7"
+            "some-type--2672975a-ce1e-4473-a1c6-0d79868930c7",
         ],
         allow_custom=True,
     )
 
     assert "some-type--2672975a-ce1e-4473-a1c6-0d79868930c7" \
-       in report.object_refs
+        in report.object_refs