The concept of dark/black being scary I'm confident pre-dates any racism towards african americans.
Also, there are a few things where black is used as positive.
- Finance; If you're "in the black", you're flushed with cash. There's some kinda dark irony here. ;)
- Rich people tend to wear expensive black clothes, expensive black cars... etc. Black is always in fashion and a symbol of wealth... which somehow, again, is ironic. ;)
The black finance thing is rather simple - in bookkeeping, a positive total is written in normal ink, which is (usually) black. A negative total is written in red ink, to make it stand out. Being in the black just means you're not making a loss, though when used colloquially, as you say, it means you are flush with cash.
Red is the color of life, of hearty blush in skin.