Dark skin means more light is absorbed, and therefore doesn't bounce back at the viewer's eyes.
the only explanation I can think of is that the rules are somehow different for light outside the visible range?
Perhaps the question is more complex because sunlight is not all in the visible wavelengths and skin is actually translucent and light reaches different level through the skin, melanin prevents parts of the light to reach deeper levels in the skin and also converts parts of light.
Melanin inhibits the above reaction, so blacks need more sun to get the same amount of vitamin D.
More details: http://www.cholesterol-and-health.com/Vitamin-D.html