blue jeans vs denim

blue jeans and denim both are nouns.

Word	Noun	Adjective	Verb	Adverb
blue jeans	Yes	No	No	No
denim	Yes	No	No	No

As nouns, blue jeans and denim are synonyms defined as:

blue jeans and denim: (used in the plural) close-fitting trousers of heavy denim for manual work or casual wear

Other synonyms of blue jeans include jeans.

blue jeans (noun)	denim (noun)
(used in the plural) close-fitting trousers of heavy denim for manual work or casual wear	a coarse durable twill-weave cotton fabric
	(used in the plural) close-fitting trousers of heavy denim for manual work or casual wear