Hard to polish how? Requires more pressure for a longer length of time like most urethane balls?
From the Storm tech description of the Pitch Black, it seems like it probably old fashioned true urethane, not a urethane blend like most balls called "urethane" these days. As such it can be VERY, VERY hard to polish.
I think you need to take the surface up in stages from the stock 1000 grit to as high as you can, not skipping any stages in between grit levels. If it's as hard as old urethane is, you need to go from 1000 to 1200 to 1500 to 2000 to 2500 to 3000 to 4000 grit, before even considering polish. You might get all the skid you need at 3000 or 4000 grit.
Personally I found only Lane#1 Secret Sauce was capable of polish these true urethane balls.