I would not consider it racist, no. Given the history of the people, the struggle, civil rights, etc, you couldn't really consider a vote for Obama by a black voter because Obama is black to be a racist act.
Also it's not just about Obama, black Americans tend to vote Democrat, as you know, regardless of the color of the candidate. Therefore a black Democratic candidate is likely to poll very well with black voters.
Regardless, Obama will get overwhelming support from black Americans, IMO.