I’ve been tattooing for a long time and I’ve worked on every conceivable shade of human skin. I’ve noticed that the most important thing isn’t usually how dark or light someone is (although that matters when you discuss tonal value and contrast) but the hue of a person, the underlying warmth or coolness of their skin.
Skin color is created by melanin, a pigment found in the upper layer of the epidermis. Tattoos lie beneath this layer, in the area between the epidermis and dermis. This placement of the ink prevents it from being shed with dead cells, by the top layer, and by being dispersed into the capillaries, in the bottom layer.
Since the ink itself lies beneath the epidermal melanin, looking at a tattoo is like looking through a tinted window. Except for albinos, everyone on earth has melanin in this skin layer. Some will have a ruddy skin tone, some a cold tone to their skin. Some will be dark, some light. Some freckled, and some smoothly pigmented.
Taking all of this into account when designing a tattoo is important. Tattoo ink is not opaque, but translucent, so you see through one tinted window, then through the ink itself. More than one factor has to be taken into account to make a great tattoo on uneven or darker skin tones.