/ blɒtʃ; NAmE blɑːtʃ/ noun
a mark, usually not regular in shape, on skin, plants, material, etc. :
He had come out in (= become covered in) dark red blotches.
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WORD ORIGIN
early 17th cent. (as a verb): partly an alteration of obsolete plotch in the same sense (of unknown origin), influenced by blot ; partly a blend of blot and botch .