Death, ambition, and darkness are the stock and trade of black mages.
Black mana offers a host of gruesome spells. Black magic can unearth the dead from their graves and make them walk again. It can spread a pestilence across the landscape and drain the life from all who live there. Black spells can curse victims in horrible and enduring ways. They can warp the mind, poison the land, and kill creatures instantly. But the price is high and the risks are many. At their best, black mages are ambitious and unashamed. At their worst, they are enslaving and devouring.
Examples of black spells are tendrils of corrupting shadow, command over the undead, and projection of crippling nightmares into the minds of enemies.
Undead skeletons and ghouls, unspeakable horrors, and diabolical demons are native to places of black mana, and they're as self-serving and treacherous as the mages who control them. Black magic isn't limited to these abominations, though—death-cultists and secret societies of assassins carry out black mages' nefarious plans under cover of darkness. Black mages control the most horrifying of creatures, but always at a price.
Each of the five colors of magic has two ally colors and two enemy colors. Black is allied with red and blue. Black and blue mana share the values of secrecy, manipulation, and desire for omniscience. Black and red mana share the values of individualism, instant gratification, and self-interest.
Black mana opposes white mana through the conflicts of darkness vs. light, corruption vs. purity, and the needs of the individual vs. the needs of the many. Black mana opposes green mana through the conflicts of death vs. life, decay vs. growth, and greed vs. abundance.