The question though is how did skin colour, or more accurately "Blackness", become a marker for "class". Without reference to chattel slavery and the construction of blackness such that an entire geographic region (Africa) could be defined not as a source labour power (in the Marxist sense of variable capital) but rather as fixed capital it is impossible to speak of the specific way in which class gets reproduced in and through reference to blackness in the US. And I suspect if one is black it is quite clear that their "blackness" is the cause of their class position precisely because their status as black came prior to their status as working class or poor. If the US were only characterized by class division then it would be true that many black people would be poor have marginal jobs and belong to the working class but as a relative percentage of the population so defined it should be no higher than for any other "group." That is if African Americans make up 15% of the population they should contribute only 15% to the categories enumerated above. Anything over that rate indicates the legacy of and objective proof of systematic racism in the reproduction of class.
Travis