The Collaborative International Dictionary of English v.0.48:

Blotch \Blotch\, n. [Cf. OE. blacche in blacchepot blacking pot,
   akin to black, as bleach is akin to bleak. See Black, a.,
   or cf. Blot a spot.]
   1. A blot or spot, as of color or of ink; especially a large
      or irregular spot. Also Fig.; as, a moral blotch.
      [1913 Webster]

            Spots and blotches . . . some red, others yellow.
                                                  --Harvey.
      [1913 Webster]

   2. (Med.) A large pustule, or a coarse eruption.
      [1913 Webster]

            Foul scurf and blotches him defile.   --Thomson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

blotch
    n 1: an irregularly shaped spot [syn: blotch, splodge,
         splotch]
    v 1: mark with spots or blotches of different color or shades of
         color as if stained [syn: mottle, streak, blotch]