Barack Obama is, of course, descended from John Punch, the very first African slave in Virginia. Facts like that demonstrate the difficulty of using this or any metric rather than simple self-identification.
He's also has his mother and grandparents from Kansas, they come from who knows of what kind of European culture.
And his speeches and books seem to suggest he's comfortable with both backgrounds.
Take this into account,along with the politics regarding the above...and it's just waaay too tricky.