One can, maybe, distinguish between the value of slavey labor and the value of the labor of sharecroppers, although I have never seen such an analysis, but I think that it can be safely said that the black population of the United States, did NOT receive as much proportionately as they gave. They were enslaved and then economically suppressed until WW2. Certain immigrants, such as the Chinese who were imported to build the Central Pacific were treated perhaps even worse. since the Exclusion Act prevented many from having a decent life with women of their own race, and their wages were less even than those Americans and those European immigrants who built the Union Pacific. But the blacks remain a special case of injustice done. That injustice did not begin to be lifted until WW2, and today, despite their relative poverty, American blacks are by earning several times more than the average white man was 1940, Much of this is owing to their own failures, but considering that the heavy weight of Segregation was not lifted until just fifty years ago, and that the playing field for them was not leveled until after 1970, when the Great Post-War boom had ended, they have some right to feel they were cheated. But nursing that resentment would be the worst thing they should do, and Obama is doing no service to them to encourage it—as I think he has.