mid-14c., "to disparage, dishonor, impair morally;" late 14c., "to damage or spoil, disfigure," 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," also "shining white."
From mid-15c. as "mar the beauty or soundness of." Usually in reference to something that is well-formed or otherwise excellent. Related: Blemished; blemishing.