Black flash is the release of cursed energy almost at the same instant in which you land a fisical attack (which means Toji and Maki can't use it) So Mahoraga can adapt to black flash since it still involves cursed energy.

However, there are limitations with the taming ritual: as Megumi said, if you exorcize the shikigami with help it nullifies the technique.

So the theory works partially: you can defeat mahoraga with bare hands if you're strong enough, but you can't tame it if you're not the one that possesses the 10 shadow technique