Leave the direct definition of IBlobStorage in place, only expand the…

… definition if necessary inside the __init__.

src/ZODB/blob.py

@@ -733,6 +733,7 @@ def temporaryDirectory(self):
         return self.fshelper.temp_dir
 
 
+@zope.interface.implementer(ZODB.interfaces.IBlobStorage)
 class BlobStorage(BlobStorageMixin):
     """A wrapper/proxy storage to support blobs.
     """
@@ -753,12 +754,10 @@ def __init__(self, base_directory, storage, layout='automatic'):
         self._blobs_pack_is_in_progress = False
 
         if ZODB.interfaces.IStorageRestoreable.providedBy(storage):
-            iblob = ZODB.interfaces.IBlobStorageRestoreable
-        else:
-            iblob = ZODB.interfaces.IBlobStorage
-
-        zope.interface.directlyProvides(
-            self, iblob, zope.interface.providedBy(storage))
+            zope.interface.directlyProvides(
+                self,
+                ZODB.interfaces.IBlobStorageRestoreable,
+                zope.interface.providedBy(storage))
 
     def __getattr__(self, name):
         return getattr(self.__storage, name)