One of the most common reasons we hear for not adding things to the site's blacklist is the cost of checking each item on every single post. While I'm not at all familiar with the mechanisms in place to perform these checks, I'm still curious if there is a way that we could modify this system in order to be more effective, in this case with tagging.
Currently, we reserve the blacklist for egregious tags that constantly keep coming back and posing a problem. However, it'd be nice if the site's moderators could maintain the list and just blacklist tags that might pose further problems, rather than keep dealing with it over and over until it finally gets blacklisted.
My ideas is to combine it somehow with the existing tag synonym system. The system for replacing a synonym tag with a master tag already seems quite efficient, and has to get run on every single post anyways. With the vast number of synonyms that exist, especially on Stack Overflow, it seems safe to say that the cost of switching out synonyms for a master is much less than the blacklist checking script.
What if we could "blacklist" tags by simply creating a synonym:
In this case, blacklisted-tag would be a reserved tag-name, similar to untagged. You can't actually use it on a question, and only a moderator could create a synonym pointing to it. Then, before a post gets checked against the site's blacklist, run the synonym switch-a-maroo and let any bad, blacklisted tags get replaced with "blacklisted-tag" and then run the blacklist like normal. Viola, you only ever have to check for one tag in the blacklist, ever, because all blacklisted tags would have been replaced with that one.
Concerns: Obviously this would still require some extra tweaking to the system to account for information display.
Synonyms directed towards the blacklist tag should not show up in the normal synonyms list. Instead, create another "blacklisted" tab on the tags page. Perhaps moderator only? Although I don't see why a normal user shouldn't be able to view the list to see what's on it - just only let moderators make changes.
The error message would somehow need to keep track of which tag(s) got remapped to the blacklist tag so that it can inform the user which one needs to be removed. Might be tricky.
Again, I claim no knowledge of how the system currently works as far as efficiency or if this is even a feasible solution. I'm just trying to throw out an idea that would not only make tag blacklisting more efficient, but also give the site's moderators the option to maintain it rather than requiring intervention from the SE team.