early 14c., "to hurt, damage," from Old French blemiss- "to turn pale," extended stem of blemir, blesmir "to make pale; stain, discolor," also "to injure" (13c., Modern French blêmir), probably from Frankish *blesmjan "to cause to turn pale," or some other Germanic source, from Proto-Germanic *blas "shining, white," from PIE root *bhel- (1) "to shine, flash, burn" (see bleach (v.)).
The order of appearance of senses in Middle English is "hurt, damage;" "impair morally, sully" (late 14c.); "mar, spoil, injure" (early 15c.); "to mar the beauty or soundness of" (mid-15c.). Related: Blemished; blemishing.