Quote Originally Posted by Lemmy View Post
Like most other instances of "Dark/Black is Evil" (black magic, dark elves, etc) in pop culture, it's probably because of darkness... You know... With humans being a primarily visual species and having bad night vision and all that.

It's only natural that over time at least some people and cultures would start associating "darkness" with "danger" and therefore "evil" and "death"... And in turn, that association would drip down on mythology and superstition.
This one is actually more simple than you all think. Black cats were thought to be the familiars of (evil) witches and are still associated with witches in modern culture. Seeing a black cat was an omen that something horrible may be afoot.