impair vs blemish

impair is not a noun while blemish is a noun.

Word	Noun	Adjective	Verb	Adverb
impair	No	No	Yes	No
blemish	Yes	No	Yes	No

As verbs, blemish is a hyponym of impair; that is, blemish is a word with a more specific, narrower meaning than impair:

Other hyponyms of impair include cloud, corrupt, defile, sully, taint, deface, disfigure.

impair (noun)	blemish (noun)
	a mark or flaw that spoils the appearance of something (especially on a person's body)

impair (verb)	blemish (verb)
make worse or less effective	add a flaw or blemish to; make imperfect or defective
make imperfect	mar or impair with a flaw
	mar or spoil the appearance of