sky vs blue air

sky and blue air both are nouns.

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

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

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

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

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