How effective is PRP in African- American patients?

The effectiveness of PRP in African-American patients depends upon the indication.  For example, it usually  works well in Androgenic Alopecia. We have had results with many traction alopecia  patients.  CCCA patients occasionally respond depending on the time the hair has been dormant.  Overt scarred areas will not  regrow. Find a PRP expert. 
PRPHAIRMD.COM 

I am not sure what problem you are wanting addressed with PRP-facial rejuvenation, hair restoration or O shot. PRP works well in African Americans. The growth factors in the PRP help with rejuvenating all skin types. I have seen wonderful results with PRP Microneedling and injections for acne scarring in African American skin.

PRP is a restorative regenerative treatment that improves the tissue it is injected into.  In the hair restoration field, it shows great improvement in healing post procedure and can also be used annually (depending on the method used) to reverse the aging process of follicles. 

Platelet rich plasma or PRP is very effective in all race or sex. However, some people has better response than others. Individual may see some results after only one treatment but it is highly recommended to get multiple PRP treatments. Results also depends on the quality of PRP. Some hair clinics may be using poor quality of PRP and that may not produce good results but if a higher quality of PRP with proper activation is used than results are very good. First response after getting the PRP hair treatment is decrease hair loss than people start to see hair growth. If someone is looking for much better results than they could add ACell or AmnioFix which could be combined with PRP. 

The hair follicle will respond the same to PRP and skin color does not matter in that respect. I would recommend you try it.