denim vs blue jeans

denim and blue jeans both are nouns.

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

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

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

Other synonyms of denim include jeans.

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