Being something less than a postracialist, Ta-Nehisi Coates of The Atlantic just remembered Chris Mathews was white
The "I forgot Obama was black" sentiment allows the speaker the comfort of accepting, even lauding, a black person without interrogating their invented truth. It allows the speaker a luxurious ignorance—you get to name people (this is what black is) even when you don't know people. In fact, Chris Matthews didn't forget Barack Obama was black. Chris Matthews forgot that Chris Matthews was white.
I'm put back in the mind of the The Wire, when Slim Charles tells Avon that it really doesn't matter that our wars are based on a lie. Once we're fighting, we fight on that lie until the end. I would submit that a significant number of white people in this country, can not stop fighting on the lie. They can't cop to the fact that they really have no standing to speak on Obama's relationship to blackness, because they know so little about black people. It's always hard to say, "I don't know." But no one else can say it for you.
This is why Obama will never be postracial—he can't make white people face the lie of their ignorance, anymore than Jimmy Baldwin could make black people face the lie of our homophobia. It's white people's responsibility to make themselves postracial, not the president's. Whatever my disagreements with him, the fact is that he is brilliant. That he is black and brilliant is pleasant but unsurprising to me. I've known very brilliant, very black people all my life. At some point that number of white people who still can't their head around our humanity will have to accept the truth: the president is black, and even if you don't quite know what that means.