Barack Obama...not "black" enough?

I really don't understand the "not black enough" mentality.

According to recent census data, blacks make up around 12% of the U.S. Population. Assuming every black person thought and voted the same (which is definitely not true), while a significant voting block, it would still not be enough to get elected if it was at the cost of alienating other races.

If I was a candidate, I would be more worried about not being mainstream enough. If Obama has any chance at all, it will be through support from moderates and liberals of all races. Liberal blacks cannot single-handedly elect a leader at the national level. The numbers don't lie.