That is an inconsistency that will be rectified in the future. BlackFlame people will tap into any power they can to forward their own plans, especially those of their mortal enemy.
Religions are only very loosely connected to magic ways. You can be stronger in Dark Way than other Ways, yet be a devout Talad follower, etc. it is the initial 'supposed' character mentality that Creation assumes of characters that sets the stats. A person that follows BF is assumed to be cruel at heart, and therefor would enjoy the pain and misery one can inflict with Dark Way.