I mean like, why each job exists and what each job was... well, what was their purpose in the past and why so few of them are, around (they are exceedingly rare compared to Classes).

Edit: ...found them, but....

“In days long past, there existed an evil clan and arcane art known as black magic—a potent magic of pure destructive force born forth by a sorceress of unparalleled power. Those who learned to wield this instrument of ruin came to be called black mages, out of both fear and respect for their gift. Yet great power served to corrupt the judgment of mortal man, and so he unknowingly set out upon the path of ruin.
Adventurers who take the black will become agents of devastation, capable of annihilating those who oppose them through little more than the force of their will.”

That description though O. O. Wouldn't expect a Warrior of the Light to, yea, be one of them.