The War of the Magi definitely includes Nym and Amdapor, but I think it is still nondescript enough to include other cities which we, as players, are not aware of.

That being said, what makes me question the origin of black mages is the fact that there are black mage tonberries in the Scholar quest: You venture inside Amdapor Keep, and there are tonberries wearing black mage robes and hats. We could infer that either black mages were present in Nym, allied with Nym, or the-merchants-which-are-now-tonberries used their time locked away in the Keep, after their transformation, to harness and practice the black magic arts.

Also, the summoning of powerful voidsent is quite possible with a group of black mages - as per the final back mage quest. But at what cost? Lives? Destruction of the land? Well...

...I certainly don't want to call forth anymore than my ahriman mount from the void, that's for sure.