blue sky vs blue air

blue sky and blue air both are nouns.

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

As nouns, blue sky and blue air are synonyms defined as:

Other synonyms of blue sky include blue, wild blue yonder.

blue sky (noun)	blue air (noun)
the sky as viewed during daylight	the sky as viewed during daylight