Because humans are diurnal, and were subject to big cat and other nocturnal animal predation in pre-historic times (a large percentage of ancient skulls in Africa show signs of big cat munchery, which makes me wonder why people have historically thought lions are proud and noble). In other words, night is when the animals who see in the dark drag you off and eat you while you sleep. Ergo, darkness/blackness is "evil". It was also when people had sex, though, away from prying eyes, so its also a symbol of eroticism, or forbidden love.
In China and Japan, though, you wear white to funerals because its a symbol of death (and never give anyone a wedding card in a white envelope). Black isn't a bad colour at all. So some of it is cultural, too.
More recently in contemporary Western fashion, black is perceived as being very edgy and cool, probably because of its slightly sinister, non-conformist essence.
Darth Vader, to whom HK refers, was undeniably evil, but that doesn't explain his popularity. He was also unfathomable and mysterious, powerful, and arrogant. And he wore black.
Can I send someone a bill for this? It took me 10 minutes to think about and write.
