The last year in which the black unemployment rate was lower than the white unemployment rate was 1930. That was also the last year in which there was no federal minimum wage law.
The Davis-Bacon Act of 1931 was in part a result of a series of incidents in which non-union black construction labor enabled various contractors from the South to underbid Northern contractors who used white, unionized construction labor.
The Davis-Bacon Act required that “prevailing wages” be paid on government construction projects– “prevailing wages” almost always meaning in practice union wages. Since blacks were kept out of construction unions then, and for decades thereafter, many black construction workers lost their jobs.