First of all we've fixed a bug where newly blacklisted tags are not removed from the system.
This would sometimes cause an error if a blacklisted tag existed and was automatically suggested in a new thread.
From 2.5.1.2 on, when a new blacklisted tag is added, existing matching tags are deleted.
Because there could be existing blacklisted tags, we've added a new batch process to remove orphaned tags.
It should only really need to be run after upgrading to 2.5.1.2.
It can't deal with regex blacklists, so you'll need to manually re-add regex blacklisted tags to have the system delete any existing tags matching the regex.
Lastly we've added a bulk import for blacklisted tags. It requires a list of tags to blacklist with one on each line. This is useful if you find a long list of common words which you don't want used as tags. I found a list of about 1,000 common English words which I blacklisted via the bulk import.