Good question. I actually didn't test that at all. I mostly added that as I saw a few mostly empty blocks written and compression lowered the total space usage a bit.
A sophisticated approach to block compression would be to build a dictionary using something like zstandard's "training mode" and a random sample of data blocks, store it (versioned) in FDB as well and reference the dictionary version from compressed blocks. That would get better compression with the relatively small blocks.
