I dated a black guy who had a tribal design from his wrist all the way up his arm going on to his neck, shoulder and back. It was very, very, sexy.
If you have very dark skin then the lighter, pastel type colours wont show up because the ink is translucent. So the darker the skin the less vibrant it will be. Rich colours should show up fine though.
The problem being with tattooing darker skin is that those with African origins can be more prone to keloid scarring. I have some keloids myself (but am white) but I can be tattooed and pierced without this scarring issue. My tattooist is a friend and he gave me some tattoo spots under my foot to check this wouldnt happen.