You might be better at making sure there are few single pip cards in black so it's harder to splash. You might then need to extend that to GW, but that might be easier said than done as it's quite a strict design constraint. You could also make the black cards require a lot of black mana or swamps.
Or only provide GW duals but no black-x duals?
Or only provide GW duals but no black-x duals?