the "base" country you start in doesn't have black magic, and each protagonist basically fails to "resist the temptation" of learning black magic after encountering it. Each time, they decide that the costs of specifically
them not learning it at that moment are too high (they might be enslaved, their country will be invaded, etc etc), and they intend to use it only "ethically". The "base" country in fact once possessed black magic knowledge but systematically wiped out any mention of it after a horrible misuse of black magic, thus leaving it vulnerable to attack many centuries later, which is a good addendum to
@DezoPenguin's point about, perhaps, magic itself not being evil. After suffering a devastating invasion, their institutions have to decide how they intend to reintegrate black magic into their society. (The series seems to answer that they have to, but it's kinda a tentative answer because all of the societies that
do have black magic in the series are.... really ethically troubling.)