As to whether or not it would be global, it would have to be. Wikia uses a central user database so it isn't possible for an individual wiki to allow/block specific usernames.
There is a global blacklist for usernames, executed by the global abuse filter/global blocking tool. You can visit User:<insert racial slur here>, add numbers after certain slurs and you'll notice that sometimes usernames containing these racial slurs are globally blocked despite not existing. When a username gets prevented by the global abuse filter (or any other extension hook, like AntiSpoof), the registration page will say "We cannot complete your registration at this time." and the response to the registration HTTP request will display the block ID the username has hit.
