The user name blacklist for new accounts currently leads to problems like this:
Basically, if a title blacklist rule exists in, say, French WP but not in English WP, and I make an account in English WP and move over to French WP, suddenly my account will not be valid. This is extremely user-hostile and fragments the single user login (SUL) system.
The benefits of any kind of highly specialized blacklisting for local edge cases are outweighed by the accumulating cruft of local rules that can trap any good faith user at any time. Wikimedia is a multilingual pool of wikis where you're required to move around wikis, especially to sites like Meta, Commons or (in future) Wikidata.