Just because more black people work in polluted areas doesn't make it racist. Unless they are forced too
The reasoning seems to be that society as a whole will care more, or less, about a problem depending on how much regard society has for the victims of the problem. If a problem does most harm to people society doesn't have regard for, then society will care less about that problem than they would if it did most harm to some other segment of the population. Thus, when society fails to attend to a problem, and the problem does its harm disproportionately to blacks, one may suspect society isn't attending to the problem partly because society as a whole is failing to respect the value of black lives. The failure to attend to the problem is presumably not entirely about whether black lives matter, but perhaps that does contribute to it to some extent. There is lots of room for disagreement as to how much it contributes.