Most Helpful Guy
Please ask for their consent and explain to them your situation. You can be as honest or as vague as you wish, just get their consent to take the picture. To think they don't have feelings on the matter or a say in the matter would point towards racism.
Most Helpful Girl
Why did you feel the need to specify black people if you're photographing white people as well? The titles drifts from the rest of the content you just gave us and in all honesty doesn't make sense.