I suppose that when a user tries to submit question with a blacklisted tag, he gets warning of this type:
sorry, the 'example-tag' tag is not allowed.
(I don't know this for sure, but I guess this is the case, since this is what I get when I try to submit a question with tags which I suspect to be blacklisted. If the precise text of warning is documented somewhere, I would be grateful for this information.)
I would like to know whether this text can be customized, i.e., if some tags is blacklisted, whether it is possible to add some explanation to the error message. (E.g. why this tag is not allowed, what tags should be used instead.)
One possible situation when having something like this could be useful is the situation described in this feature request at meta.math.SE: Disambiguation pseudo-tags. (But I have no doubt similar needs arise at other SE sites, too.)