KW: people are not generally discussed in terms of a recipe: half a teaspoon of this, half of that....
Many, many people of dual parentage call themselves black (and, really, you know, they are brown...), they are a sgade of black, albeit a lighter shade than others. And, as such, subject tthe same level of racism. Racists don't appply racism in calibrated measures according to exact skin tone.
Of course he is the product of a mixed family background. You can be of mixed race, still 'black' (i.e brown) and still acknowledge your heritage as regards half your family in other ways.
I am white - I don't feel at all slighted or insulted that BO himself calls himself black. If as a white woman you have a child with a black man (DS is mixed race) you know that your child is probably going to have brown skin. Why anyone would run around after him going 'but hat about meeee, look, he's toned downbecause of meeeeeee, call him half white like meeeee', I don't know. Unless they were v insecure.
And it's VERY VERY important that he chooses to cal himself black as president precisely because of the 'one drop' rule that defined people as black and precluded so many generations of people who looked like him from equality.