There was a research study done and published in the British Journal of Dermatology that assessed the pH differences within the different layers of white caucasion skin and black African-American skin.  The study measured 10 whites and 8 African Americans to observe if there was a difference and what those differences may be.

The study was set up to measure the water loss from black skin as compared to white skin (but we already know the result of this study dont we.... can you say "ashy knees").  It was found that although the pH of the skin is typically measurednear a pH of 5, conversely African American skin was measured be be quite lower.  

It was also noted that African American skin, as you get lower levels of the stratum corneum depth showed significant decreases in pH respect to how low from the top layer of the skin was being measured.  

It was also confirmed that the superficial layers of African American skin were more prone to water evaporation and a significant difference in pH in African American skin in relation to their white counterparts.  

Skin is not the same hence African Americane should be using skin products that are specifically formulated for the science of African American skin pH rather than products formulated for other less pigmented skin types.