wild blue yonder vs blue sky

wild blue yonder and blue sky both are nouns.

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

As nouns, wild blue yonder and blue sky are synonyms defined as:

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

wild blue yonder (noun)	blue sky (noun)
the sky as viewed during daylight	the sky as viewed during daylight