Here's a weird result Al pointed out from yesterday's NYT race poll. They asked "Just as your best guess, about what percentage of all Americans are black: less than 10%, between 10 and 20%, between 20 and 30%, between 30 and 50%, or more than 50%?." You get these ranges:
White: 1 21 33 33 8 5
Black: 4 24 26 24 17 4
In short, blacks and whites are both massively overestimating the number of black people in the country. That's interesting. And I also think it's interesting how little racial divergence you see in the answers here. My guess would have been that thanks to residential segregation blacks tend to overestimate the proportion of African-Americans in the population while whites would underestimate it. But that's not how it goes.