years vs blue moon

years and blue moon both are nouns.

Word	Noun	Adjective	Verb	Adverb
years	Yes	No	No	No
blue moon	Yes	No	No	No

As nouns, blue moon is a hyponym of years; that is, blue moon is a word with a more specific, narrower meaning than years:

Other hyponyms of years include month of Sundays, aeon, eon, year dot.

years (noun)	blue moon (noun)
a late time of life	a long time
a prolonged period of time
the time during which someone's life continues