1. Blacklist whatever 2 BG's you want to blacklist (mine are IoC and AV/SotA).
2. Get "failed to blacklist bla bla" error.
3. do a /reloadui
4. Uncheck the 2 BG's, and check them again - this time you WONT get the "failed.." error, and you WONT get into the blacklisted BG's
After my initial posts, I got the issue myself. The part about ticking them, unticking and ticking them again isn't working. I thought it was my UI but guildies with same UI don't have the issue. Then I'm thinking it must be an add-on. Will try the reload thing.
I opened a ticket some time ago, cause I started getting the issue right after 5.1. The reply I got was "We're aware of the problem".....