You can think of it as a 5-mana phasing effect most of the time
It shows that Wizards of the Coast can come up with effects that are simultaneously powerful in small doses while overwhelming in large ones
They are Vrondiss, Rage of Ancients (a Dragonborn Barbarian, leading the red/green Draconic Rage deck), Prosper, Tome-Bound (a Tiefling Warlock, helming the red/black Planar Portal deck), Sefris of the Hidden Ways (a Genasi Wizard, leading the White/Blue/Black Dungeons of Death deck), and, finally, Galea, Kindler of Hope (a Half-Elf Paladin, topping the White/Blue/Green Aura of Courage deck)
Black magic exists in an interesting place in Kamigawa: Neon Dynasty , as it seems to primarily want to cooperate with blue for ninjutsu, but any good Magic set also needs its colors to stand independently
Certainly Magic has created a number of horror-infused planes, like Innistrad, New Phyrexia, and Duskmourn, but letting fantasy artists loose with the card designs and color identity in black has generated quite a body of really messed up stuff