My Cousin Has An Extreme Itch And Skin Irritation That

My cousin has an extreme itch and skin irritation that she can not stop scratching. Her skin had turned black on her stomach and back etc. she is an African American with reddish medium brown skin. Her skin has turned black. What are some allergies or medical problems that can cause this. She has had a lot of confusing diagnosis and no solution to her problem. Can't you guide us in the right direction please
Cathy