How does UV affect skin colors in dark-skinned people?
There is a journal article almost exactly about the title question, Mechanisms of Skin Tanning in Different Racial/Ethnic Groups in Response to Ultraviolet Radiation Journal of Investigative Dermatology (2005) 124, 1326–1332.  

However, the body of the question assumes that eumelanin changes color.  Instead, the article finds that melanin gets closer to the surface of the skin, and the degree to which this effect occurs varies with race/ethnicity.