If you are an American and you are part black, then dude, you are black. It's not his fault that America sees it this way. Can you imagine the ridicule people would throw at him if he continued to say, "Yeah, but I'm half white!"
I would feel sorry for the dude were it not for the fact that he, without family money, has risen to the highest ranks in American society and politics and has become a symbol of unity and hope for the entire country. ALL THAT in the face of the prejudice that exists in the minds of Americans.
He HAS, actually, MANY TIMES, spoken of his white mother from Kansas and the white grandparents who helped raise him. Most of his black influence came later when he worked in Chicago. I think it is a thing of real importance for a person who grows up in a white world but sees a black self in the mirror every day to find his way among black people at some point. This is exactly what Obama did, and I respect him for it.