@Xon ?Are you using the "Restrict addition of tag to threads in these forums" feature for the tag?
I get the same "Some tags are blacklisted" error if I try to use a tag restricted to a specific forum in a different forum. It would be helpful if there was a different error message in that situation.
Also, I think I found a bug @Stuart Wright. I have a tag restricted to a series of forums. But I realized today that people are able to add forum-restricted tags even outside of that forum, but only when doing it while starting a brand new thread. If they edit the tags on an existing thread, than the tag restriction holds.