Once again: Who is Black?

So I'm at this Black political convention in Chicago and there is this cat there who appears to me to be White, but I am not certain.  He also had an accent which sounded British to me. At one point another attendee indicates to me that this individual should not have been allowed up in the spot.  As it turns out this man who appears to be White is Dennis Brutus, who is an emeritus professor of Africana studies at Pitt and a well known Black activist, poet and political prisoner (alonside Mandela @ Robben Island).  This was the first time I had heard of him, so I stood mute as his presence was questioned.  But this experience was very memorable because it really brought to mind how confusing the notion of race is and how quickly race and the illusion of race can produce knee-jerk reactions. So my ultimate question is this:  can a "Black" person who passes for "White" be "racist" or not???