It's the last part that doesn't make sense. If black Americans strongly support Obama's positions, how on earth does that make the "overall" support deceptively high? I mean, black people are part of America "overall," right? George Bush enjoyed very strong support among Southern white evangelicals. Did that mean Bush's "overall" support was deceptively high?
I don't like throwing inflammatory words around on issues of race. But there's got to be some label for this subconscious discounting of black votes -- and the implicit overvaluing of "real" (i.e., white) America. It's just weird.