Whereas geists associated with other colors of mana might be benevolent or neutral, black-aligned geists are almost always dangerous and malevolent.
As far as the benefit of the rest of Magic is concerned, gold cards in Legends were executed perfectly. They got all the excitement a designer could hope out of a splashy new mechanic without using up any of the valuable design space. Truly amazing.
--Aaron Forsythe's Random Card Comment on Kei Takahashi