I haven't used blobs myself.
I'm not sure if an image blob is in base64. In email, images can be base64.
There's a db command in api called db_blob_get: https://openacs.org/api-doc/proc-view?proc=db_blob_get&source_p=1 It states it's 'PostgreSQL only.' but the source shows code for oracle as well. Maybe it's buggy.
Maybe this thread has something useful: https://openacs.org/forums/message-view?message_id=88888
