I don't think Verin is Mesaana, but I do think that she is the head of the Black Ajah. She who always appears hooded and only Galina and a few more knows the identity of. I think it is quite clear that she is black. She breaks the three oaths when she says that Moiraine sent her in "The Great hunt". There are also a few mutilations that occurs whe she is around. (Namely Barthanes and that philospher in Cairhien that did research for Rand.) If you reread the whole series and think of her as black all the time there is no real evidence against it.