Five little words — "I forgot he was black" — have exposed a contradiction in the idea of a post-racial nation. The comment came from MSNBC host Chris Matthews after President Barack Obama's State of the Union speech on Wednesday. "He is post-racial, by all appearances," the liberal host said on the air. "I forgot he was black tonight for an hour. You know, he's gone a long way to become a leader of this country, and past so much history, in just a year or two. I mean, it's something we don't even think about." Matthews meant it as...