I ran into some issues trying to delete a custom metadata key from a storage object. I expected the following to work, but did not result in the key being deleted:

# blob has metadata with key 'foo'
new_metadata = {}
for x in blob.metadata:
    if x != 'foo':
        new_metadata[x] = blob.metadata[x]
# new_metadata is now a copy of the metadata without key 'foo'
blob.metadata = new_metadata
blob.patch()
# The blob's metadata still has key 'foo'

The Rest API indicated to send a value of JSON null associated with the key to perform a deletion, but that doesn't seem to happen when issuing a blob.patch with a metadata with a key removed.

As a work around, I created a copy of the desired metadata, deleted all of the blob's metadata (which does work), then updated the metadata to be the desired metadata.

new_metadata = {}
for x in blob.metadata:
    if x != 'foo':
        new_metadata[x] = blob.metadata[x]
blob.metadata = None
blob.patch()
blob.metadata = new_metadata
blob.patch()