It depends on the person, but some colors get "lost" as they're viewed through the layers of darker skin. Black allows for more lasting contrast on dark brown skin.

As far as not seeing the tattoo, sometimes it's just a case of crappy artistry. (Where black is not clear bc the ink isn't properly applied/the skin is oversaturated and damaged)

I do admit I always say I'd look like a coloring book of I had very pale skin though! (As I would opt for really vibrant full color tattoos) anyhoo be well