"The problem I have with this theory is that it fails to explain how African Americans' labor suddenly changed from extremely desirable to extremely undesirable to white capitalists. A few laws get passed giving a little bite to African Americans' formal equality and all the sudden no one wants to hire black people? That sounds unlikely to me given the long and sordid history of the effective exploitation of black labor,"
But as I understand it, the theory is not that excessive wage demands are the reason that black labour becomes superfluous. 'Wages are the dependent, accumulation the independent variable', says Marx. In other words, it is not excessive wage demands that make labour superfluous, but a slower rate of investment that reduces the demand for labour. (The lower rate of investment due to the falling rate of profit.)
No doubt there are many other variables before the rate of accumulation gives rise to an increase in the prison population.
In any event, I am not sure this holds up since recent trends in capital accumulation, I believe are towards 'extensive growth', or 'job-rich growth'. This is not very up-to-date but I think the US workforce grew by 20 per cent between 1986 and 2001 and net immigration increased. So surplus population does not seem a good explanation of increased imprisonment