They dont even need to do that. Just because the sunwell is fixed doesn't make the blood elves perfect. They're not just goody two shoes now. I think people make a hasty generalization about that. Felblood elves could quite simply just be corrupt blood elves who are even more demonic than their "normal" kin (who are already slightly felsworn). It would make perfect sense for a lot of the blood elf mages and especially warlocks.