Whispering /bl to the character used to work up until a couple of days ago on bots/other players but no longer blocks them. I couldn't find any mention of this being removed but it clearly doesn't work anymore; other people are reporting that it too doesn't work and the only way to block people is through blocking their XBL gamertag.

To be clear this is Ship 1 NA and to be doubly clear the in-game blacklist function WAS 100% working up until at-least last maintenance but no longer functions.