sky vs blue sky

sky and blue sky both are nouns.

Word	Noun	Adjective	Verb	Adverb
sky	Yes	No	Yes	No
blue sky	Yes	No	No	No

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

Other hyponyms of sky include blue, blue air, wild blue yonder, mackerel sky.

sky (noun)	blue sky (noun)
the atmosphere and outer space as viewed from the earth	the sky as viewed during daylight

sky (verb)	blue sky (verb)
throw or toss with a light motion