It seems the blacklist is on the your side not the person you are trying to blacklist ..

So instead of the person not being able to send the message, they are sending it as normal and it just isn't being displayed at your end.

It's one of those bizarre SE what are you doing things, wouldn't surprise me if they did it this way because they can't program it the other way due to some oversight when the chat system was designed. (tells in dungeons for example)