One of black's unique properties is its corruptive nature. You can add black to any color and it pretty much just becomes a darker, edgier version of that color. Look at the set Torment. It just taints and slowly kills/transforms what its gotten into.
A few reasons for this: Black has a pretty wide range of design space, because black can do nearly anything if you're willing to pay life for it. So it makes mechanical sense added to a lot of colors.
Black's also got a magnification theme. The more black you have, the better black gets.
One can say that black isn't necissarily evil, and there have been a few black protagonists and plenty of villains that were totally not black at all; mono-whites and mono-blues, especially. But when its an effect that's intentionally meant to creep into you, then its pretty fucking evil.
Remember, its not just that Garruk's using black mana. Its that he's been CURSED with a specific magic power that was probably just supposed to kill him. He turned out to be a bad enough dude to instead adapt it and make it part of his nature.