I think the block cache is an in-memory copy of the SQLite Block table meant to shorten a lookup whether a data block read from a file being backed up was backed up already. If so, it doesn’t require another backup.
https://forum.duplicati.com/search?q=use-block-cache found a few mentions, with one of the better ones at:
I have just tested with a small backup (~2000 blocks), and a block lookup cache had a tiny positive effect on the backup speed. But it is possible that this speedup is much more pronounced with larger backups, so I made a canary build with the option
--use-block-cache. If it turns out that it really does improve performance without ridiculous memory overhead, I will convert it to a--disable-block-cacheoption.
v2.0.2.6-2.0.2.6_canary_2017-09-16
Added an experiemental --use-block-cache flag to test performance potential