The Collaborative International Dictionary of English v.0.48:

Scumbling \Scum"bling\, n.
   1. (Fine Arts)
      (a) A mode of obtaining a softened effect, in painting and
          drawing, by the application of a thin layer of opaque
          color to the surface of a painting, or part of the
          surface, which is too bright in color, or which
          requires harmonizing.
      (b) In crayon drawing, the use of the stump.
          [1913 Webster]

   2. The color so laid on. Also used figuratively.
      [1913 Webster]

            Shining above the brown scumbling of leafless
            orchards.                             --L. Wallace.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Scumble \Scum"ble\, v. t. [imp. & p. p. Scumbled; p. pr. & vb.
   n. Scumbling.] [Freq. of scum. [root] 158.] (Fine Arts)
   To cover lighty, as a painting, or a drawing, with a thin
   wash of opaque color, or with color-crayon dust rubbed on
   with the stump, or to make any similar additions to the work,
   so as to produce a softened effect.
   [1913 Webster]