My 5.x site uses block_cache module - so it's got a cache_block table. This makes update_fix_d6_requirements since there's no table_exists check in there.
I see two options:
1. Document this in upgrade instructions somewhere - either in the block_cache module, or changelog.txt or something that you'd need to uninstall first (like update status)
2. Drop the table if it exists then make it again.
The issue with 1. is if there's a 6.x version of block_cache - we can only lose everyone's cache settings. With 2. we'd have to know whether the block cache module in 6.x is going to use the core block_cache table and put it's own ui into it.