tarnish vs darken

tarnish is a noun but darken is not a noun.

Word	Noun	Adjective	Verb	Adverb
tarnish	Yes	No	Yes	No
darken	No	No	Yes	No

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

tarnish: make dirty or spotty, as by exposure to air; also used metaphorically
darken: tarnish or stain

tarnish (noun)	darken (noun)
discoloration of metal surface caused by oxidation

tarnish (verb)	darken (verb)
make dirty or spotty, as by exposure to air; also used metaphorically	become dark or darker
	make dark or darker
	tarnish or stain