You already know your skin isn't black. In fact, 'black' skin is the most colorful in the entire shade spectrum. So how does knowing that help you? Mostly, it helps you understand why you look ashy whenever those who are inexperienced with deeper skin tones apply your foundation.
It took years of research for us to develop a comprehensive system that guarantees a match, which is why those just breaking into the makeup field / only experienced with 'white' skin resort to what works for lighter skin tones (which just doesn't work for your depth of color) . . .