I expect that everyone has heard the "Blacks are lazy" slander. I think a single economic principle has two aspects that may explain its genesis: if everything else is the same, people will prefer leisure to work. In other words, everyone is lazy. So why do blacks get picked on? Two reasons: First, racism rewards blacks less for work, giving them less incentive to work hard. Second, that the difference between the work output of a slave versus the same person as a freedman could be perceived as laziness. Even the smallest effect would be picked up by a racist looking for reasons to hate blacks.
It is a fact that blacks are paid less for the same work as whites. Black unemployment is also higher than white unemployment. Racists no doubt think there is one and only one explanation: that blacks work less hard, create less value, and their continued employment can only be justified by less pay. It's much more likely that racism is the cause of "blacks are lazy".
Anecdotal evidence is always suspect, but it can be useful for what it does not show. I, myself, know of no blacks who could remotely be called lazy. A 60-hour work week, a house on the North Shore, and daughters in Princeton and Williams is not evidence of laziness.
I cannot recall where I read this, but freed slaves worked less hard upon receiving their freedom. This is predictable since a slave owner puts the highest value on the work output of a slave, whereas the slave values leisure highest (if all else is the same). Of course, all else was not the same. The slave-holder was free to use violence and imprisonment against the slave, whereas the slave only had underwork and escape.
Note that I'm not saying that blacks actually are lazy. I'm saying that people pre-disposed to find differences between themselves and others based on race (that is, racists) are comforted by the perception that blacks are lazy. It would take very little evidence to convince them of anything bad about blacks, and very large evidence to convince them of anything good. For example, a racist will generalize from a single black person resting on his shovel to thinking that all blacks are lazy. And once a racist starts believing bad of blacks, those blacks get paid less, those blacks want to work less, and you have a vicious cycle. Even if the effect is small in time, space, or magnitude, a racist will pick it up and continue to believe that blacks are lazy.
Rather than end on that depressing note, I'll end on an even more depressing note: I don't think there's anything to be done about it other than to wait for racists to die out.