We don't always blacklist problematic tags or terms, but when we do it's because they've become such a nuisance on a site that we can't control them any other way. Vague / meta tags that keep coming back, domains or urls that we really don't want links to, etc.
When someone tries to use a blacklisted term, the editing UI is good about noting exactly what is disallowed... But not so great at explaining why:
So folks encountering such blacklists either make some trivial change to the input to allow it to go through, or come to meta to ask why it's not allowed. Neither of these actions is quite ideal though...
Just because a change allows the post to avoid the blacklist doesn't mean it's the sort of change we were hoping to inspire. Using URL shorteners to get around URL blacklists is a prime example, but there are even worse unintended consequences.
Asking about it on meta is good, but time-consuming and... The meta discussion that prompted the blacklist almost certainly already exists!
It would be nice if we could include specific text - and links - to be displayed in the error message for a blacklisted entry:
This would also enable something else we normally try to avoid: blacklisting unwanted tags before we've finished cleaning them up. By providing specific alternatives in the error itself, it would be much less painful for editors unaware of the cleanup effort while preventing its use on new posts while the cleanup effort was ongoing.