The term “Black Magister” is the phrase used by the Orders and witch hunters to describe Magisters who either go rogue, turning their backs on the Articles of Imperial Magic, or who knowingly utilise Dark Magic, or both. This is not a common occurrence, but a few Black Magisters are infamous throughout the Empire.
Black Magisters do not have to have embraced the Chaos Gods, nor must they have begun studying the arts of Daemonology or Necromancy. All it takes is for them to flee their Order and refuse to practice and respect its traditions or intentionally and persistently experiment with more than one Wind of Magic at a time. These traitorous acts will earn the Magister the relentless hostility of his prior Order.
Black Magisters know and accept they have disobeyed the Articles of Magic and have broken faith with their Order. They have been instructed from day one as Apprentices that breaking with their Order is the most terrible and illegal act possible and that using Dark Magic is insanely dangerous. These acts are against the spirit and letter of the law and are morally lacking.
So if a Magister takes the rare and extraordinary step of starting to use these malign energies, he would either willingly embrace his new title of Black Magister or reject any title whatsoever. Whatever the case, Black Magisters tend to look down on their old brethren as narrow-minded, misguided fools who willfully hold back from exploring the true nature and power of magic. These renegades tend to look even more starkly on self-taught whom they consider ignorant and trivial dabblers worthy only of contempt and destruction.
Black Magisters who do not flee their Order and practice their dangerous and illegal arts in secret would be among the most subtle and powerful of all Magisters to be able to do so without being discovered. They would probably need divine or daemonic aid to hide their corruption from their peers if they reside in the College buildings of Altdorf (as was the case with that vilest of all traitors, Egrimm van Horstmann).
Source
- Warhammer Fantasy RPG 2nd ED -- Realms of Sorcery (pg. 68-69).