It should be possible to have an index that says "this encrypted blob belongs to this particular snapshot". It introduces a "plain text" component of course, in that the snapshot information (name/date or whatever) is readable.

But then, I assume the way it is written, there are some blobs that belong to multiple snapshots, and some bits of blobs that belong to one snapshot but not another etc.

So given the implementation, it may not be possible, but it's still a drawback of this particular implementation that people should be aware of.