Aluminum Polishing

Do you mean castings? If so, the black spots are most likely porosity in the casting that are picking up the dirt and grit from the polishing process. If that is the case, your only cure will be eliminating the porosity. I'd start by revisiting the casting supplier chosen and the maximum porosity limits placed on them.

thanks for response, iguess i will have to grind the pits out and repolish.

Require a surface condition prior to polishing conducive to provide the finish quality as specified.

what type of conditioner do you recommend

He means specify a maximum porosity allowable. Any castings that are too porous are rejected. Continuing to buff in an attempt to buff out the porosity is likely merely to expose other porosities in the process.