Per discussion today, we should rollback #263 and document that users should use cloud storage for blobs instead. Quickly scanning the code, another option would be to leave #263 in place but to document that it shouldn't be used for large byte strings and to potentially raise an error over a certain size. ISTM the code would fail if we tried to insert a blob that is large enough (10MB?).
True blob support is really an artifact of the old datastore implementation (just like GQL) which predates Cloud Storage.