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]

Moby Thesaurus II by Grady Ward, 1.0:

167 Moby Thesaurus words for "blotch":
   band, bar, bedarken, besmirch, bespangle, bespeckle, bespot,
   birthmark, black, blacken, blackwash, blaze, blaze a trail,
   blemish, bloodstain, blot, blur, brand, caste mark, chalk,
   chalk up, charcoal, check, check off, checker, checkmark, cicatrix,
   cicatrize, cork, cut, dab, dapple, dappledness, dappleness, darken,
   dash, daub, define, delimit, demarcate, denigrate, dinge, discolor,
   discoloration, dot, dottedness, earmark, ebonize, engrave,
   engraving, eyesore, flake, fleck, flick, flyspeck, freckle,
   freckliness, gash, graving, hack, harlequin, hatch, impress,
   imprint, ink, iris, jot, lentigo, line, macula, maculate,
   maculation, macule, make a mark, marble, marbleize, mark, mark off,
   mark out, marking, melanize, mole, motley, mottle, mottledness,
   murk, nevus, nick, nigrify, notch, oversmoke, patch, pencil,
   pepper, point, pointillage, pointillism, polka dot, polychrome,
   polychromize, prick, print, punch, punctuate, puncture, rainbow,
   riddle, scar, scarification, scarify, score, scotch, scratch,
   scratching, seal, seam, shade, shadow, smear, smirch, smoke,
   smouch, smudge, smut, smutch, soil, soilage, soilure, soot,
   spangle, spatter, speck, speckle, speckliness, splash, splatter,
   splotch, spot, spottedness, spottiness, sprinkle, stain, stamp,
   stigma, stigmatize, stipple, stippledness, stippling,
   strawberry mark, streak, striate, stripe, stud, taint, tarnish,
   tattoo, tattoo mark, tessellate, tick, tick off, tittle, trace,
   underline, underscore, variegate, vein, watermark