Not many responses on your question ... we don't have a lot of portrait artists and I haven't seen any around recently.
My own efforts at skin tones have been disasters, so not much help from me. Best I can do is point to some resources.
There is a portraiture forum on WetCanvas and it has a classroom. I quick browse of the classroom thread titles shows at least one thread devoted to skin tones. Here's a link to the classroom. http://www.wetcanvas.com/forums/forumdisplay.php?f=226
A Google search for: mix African American skin tones in acrylic will turn up several links to threads on WetCanvas as well as YouTube videos.
Good luck and please post the painting. Yet another in the long line of musicians, going back to the nineteen sixties, that have left us too soon.