There are a couple of questions relating to Boo, and they're tagged with scripting.
Can we re-tag them with boo, or the tag has been removed for a reason not written here and should not be re-introduced?
There is no boo simply because noone has created one.
If I had to speculate, I would say that the absence of this tag may have actually led to users using more generic tags, such as scripting. This seems to be further supported by the fact that all scripting questions that reference "boo" were created by new users.
The logical thing to do, here, is simply create a boo tag. It is a tag pertaining to a specific programming language that is used in game development, no matter how uncommon. There is no reason why such a tag would be blacklisted, and it isn't.