As I remember the explanation: Dark skin means more melanin in the skin. Melanin blocks the UV light that the body uses to synthesize D vitamin. So white people in north are better suited for utilizing the scarce sunlight for D vitamin production and black people in the south are better protected against the skin cancer risk caused by the UV radiation.

I wouldn't dismiss this so quickly. It is widely accepted and is also part of what is evaluated when the doctor recommends or not that you give extra vitamin D to your child, having white skin means you need to spend less time in the sun to have sufficient vitamin D and not require supplements.

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.

I'm guessing the absorption they are talking about happens below the pigment level.

UV-B sunlight strikes a substance on the skin called 7-dehydrocholesterol converting it into vitamin D3. The vitamin D created by this reaction is then reabsorbed by the skin and enters the bloodstream. Apparently some new research indicates reabsorption may take up to 48 hours!

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

Well the advantage of increased melanin in the skin is protection from UV damage. Perhaps this is what they are referring to.