blue sky vs wild blue yonder

blue sky and wild blue yonder both are nouns.

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

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

Other synonyms of blue sky include blue, blue air.

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