The current state is, if enwiki blacklists A but dewiki doesn't, a user can register the name A on dewiki, then autocreate the name A on enwiki. Ideally, the name can exist on dewiki, but cannot be autocreated on enwiki.
I tried to implement this in r67347 and it's CentralAuth-specific (not really sure whether this is good). Besides, this can actually work only after bug 24755 is fixed.
Assigning this bug to myself, but nothing really need to be done after bug 24755 is fixed, unless the "CentralAuth-specific" feature needs reconsideration.