Well, there should at least be a message when you try to add the synonym that you’re adding something that is blacklisted. Changing the color of the added synonym or something might be good too.
@TofuguJenny thoughts?
I don’t really see how it can be argued it’s acceptable, but I guess it’s the user’s prerogative to override the system if they want to (if, for instance, the same spelling had the right meaning in a different language)