The Stack Exchange team has the ability to blacklist a tag even when the tag still exists. With the combination of Shog's new tag-specific popup messages, some tags which are blacklisted now have messages that accompany why they are blacklisted. The tag is still kept around for those questions which have historical significance.
Sometimes, though, those questions are pretty terrible. Most notably, they have really bad titles that we'd like to clean up. Problem is, since the tag is blacklisted, we can't successfully make any edits on the question itself without removing the tag from the question. But that kind of defeats the purpose of this feature, since it is meant to keep the tag alive as a category that is no longer on-topic, for these historical questions.
I've thought of a few possible solutions to this:
Give moderators the ability to edit the question and ignore the tag blacklist as long as the tag was already on the question.
Let moderators add a blacklisted tag to a question if the tag still exists on the site (hasn't been completely destroyed yet). Basically, any tag that exists for historical purposes.
If those two are too complicated to implement, just let moderators use blacklisted tags. (Do you trust us enough not to re-invent the homework tag?)
Note: This is a request for the privilege to be given to moderators only, not all users in general. Just for clarification. This would pretty much only be used on historically locked questions that can only be edited by moderators anyways.