Who are you referring to by 'black' people? People who you designate as 'black' despite the fact that they're not? If so, I can't see why I would want to ask someone whose skin colour isn't black if he/she agrees with me that their skin colour isn't black. It's not a question of opinion, it's a question of fact. Black is a colour. Your skin is either black or it isn't.
If you mean people whose skin colour actually is black (which is a minority of those to whom the term 'black' is applied to), again I can't see why I would want to ask them if they agree with me that they are black, since unless they're blind they presumably would agree, and in any case again it wouldn't be a matter of opinion, it's a matter of fact.
If you mean, should I run this by people who think that they ought to be described as black, even though their skin colour isn't black, and who describe themselves as such, then no, I don't see any need to 'run it' by them. If they want to maintain that they are black, when that is factually not the case, then so be it, but it doesn't mean I have to concur.