scum·ble 
(skŭmbəl)
(skŭmbəl)tr.v. scum·bled, scum·bling, scum·bles
1. To soften the colors or outlines of (a painting or drawing) by covering with a film of opaque or semiopaque color or by rubbing.
2. To blur the outlines of: a writer who scumbled the line that divides history and fiction.
n.
1. The effect produced by or as if by scumbling.
2. Material used for scumbling.
[Possibly from SCUM.]