Given two identical x-ray scans (typically 15 mrem for a dental x-ray) to people of differing skin colors, the amount of x-rays absorbed is going to be the same; the difference will be where the x-rays are absorbed.
Melanin, the stuff that gives you the pigment of your skin, can diffuse or absorb more ionizing radiation in order to protect the cell's DNA from being damaged. So people with more melanin in their skin would probably absorb more in that region-although I suspect that it would be a very trivial amount. But, for the sake of argument, lets say that more is absorbed in the surface skin area than in the target region (in this case your jaw bone/teeth). This could potentially lead to a poor resolution and exposure of the film which would lead to the doctor wanting to boost the intensity of the x-ray slightly to obtain a better resolution exposure. If this were the case then, yes, the darker skinned person would then receive more absorbed dose of x-rays than a lighter skinned person. I highly doubt this though considering the mass attenuation coefficients of x-rays for added melanin. With the technical advances of radiation imaging and x-rays, I would say that the possible absorbed dose increase would be very negligible and would not cause any significant increase of the average annual exposure of 360 mrem. The heavily sun-taned person would want to worry alot more about the damaging UVB-UVC rays than the infrequent x-ray scan!
