Fix #9750: Respect blank=True when applying uniqueness constraints

rest_framework/serializers.py

@@ -1527,18 +1527,22 @@ def get_uniqueness_extra_kwargs(self, field_names, declared_fields, extra_kwargs
                 default = empty
 
             if unique_constraint_name in model_fields:
-                # The corresponding field is present in the serializer
                 if default is empty:
-                    uniqueness_extra_kwargs[unique_constraint_name] = {'required': True}
+                    if getattr(unique_constraint_field, "blank", False):
+                        uniqueness_extra_kwargs[unique_constraint_name] = {
+                            "required": False,
+                            "allow_blank": True,
+                        }
+                        if getattr(unique_constraint_field, 'empty_strings_allowed', False):
+                            uniqueness_extra_kwargs[unique_constraint_name]["default"] = CreateOnlyDefault('')
+                    else:
+                        uniqueness_extra_kwargs[unique_constraint_name] = {"required": True}
                 else:
-                    uniqueness_extra_kwargs[unique_constraint_name] = {'default': default}
+                    uniqueness_extra_kwargs[unique_constraint_name] = {"default": default}
+
             elif default is not empty:
-                # The corresponding field is not present in the
-                # serializer. We have a default to use for it, so
-                # add in a hidden field that populates it.
                 hidden_fields[unique_constraint_name] = HiddenField(default=default)
-
-        # Update `extra_kwargs` with any new options.
+
         for key, value in uniqueness_extra_kwargs.items():
             if key in extra_kwargs:
                 value.update(extra_kwargs[key])