"a spot," especially a large irregular spot, as on the skin, c. 1600, perhaps an extension of blot (n.) by influence of botch or patch. Also from c. 1600 as a verb. Related: Blotched; blotching.
Entries linking to blotch
blot n.
late 14c., "a spot or stain of ink;" also "a moral stain or blemish, a disgrace, a sin;" a word of uncertain origin. Perhaps from Old Norse blettr "blot, stain," or from Old French blot, variant of bloc "block." The Middle English Compendium compares, hesitantly, Old French blo(s)tre, variant of blestre "a boil." It is attested from 1570s as "any black or dark patch."
blotchy adj.
"disfigured with blotches," 1799, from blotch (n.) + -y (2). Related: Blotchiness.