Middle English blemisshen, to damage, injure, sully, from Anglo-French blemiss-, stem of blemir, blesmir, from Old French, literally, to make pale by wounding, of Germanic origin; akin to Old High German blasros horse with a blaze, Old Norse blesi blaze
verb
to spoil by a flaw
noun
a noticeable imperfection; especially : one that seriously impairs appearance