I've been told that black rubbers can be a cause of grotty playfields. Especially if you're playing it heaps. 
Is the cab under the playfield dirty too? You might be getting a bit of dirt migration from underneath maybe.
Black rubber is only useful to lazy ops.. Also doesn't have the same bounce quality as white.
Not to mention how awful it looks on EM and SS games.
Guess you could say i'm not a fan of black rubber.
