blemish vs birthmark

blemish and birthmark both are nouns.

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

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

blemish: a mark or flaw that spoils the appearance of something (especially on a person's body)
birthmark: a blemish on the skin that is formed before birth

blemish (noun)	birthmark (noun)
a mark or flaw that spoils the appearance of something (especially on a person's body)	a blemish on the skin that is formed before birth

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