sky vs wild blue yonder

sky and wild blue yonder both are nouns.

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

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

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

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

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