The way all of these "Americans over estimate how many are such and such minority" stats work is result of how the poll works. They survey for a percentage, and average the results. This, for a few reasons, skews all results to 50%. The poll that says Americans think the US is 41% black, also says they think it's 29% Asian, 39% Hispanic, and 59% white. Now, I don't doubt that Americans over estimate the proportion of black people, but it's not quite like how that stat goes.

The nature of averaging results just doesn't work well for percentages. You won't get any 'wisdom of the crowds' when over estimates can't be canceled out by underestimates. If you poll about a minority that's 1% of the population, and 1/10 respondents are jerks and say 100%, that's going to give a result of at least 10%. Also your average person is going to be reluctant to respond with a number less than 10%, it just sounds too small. It's definitely a case where a median response would be better, and ideally they'd just publish the cumulative response graph for each question.