blue jeans vs pair of trousers

blue jeans and pair of trousers both are nouns.

Word	Noun	Adjective	Verb	Adverb
blue jeans	Yes	No	No	No
pair of trousers	Yes	No	No	No

As nouns, pair of trousers is a hypernym of blue jeans; that is, pair of trousers is a word with a broader meaning than blue jeans:

blue jeans: (used in the plural) close-fitting trousers of heavy denim for manual work or casual wear
pair of trousers: (usually in the plural) a garment extending from the waist to the knee or ankle, covering each leg separately

Other hypernyms of blue jeans include pant, trousers, workwear.

blue jeans (noun)	pair of trousers (noun)
(used in the plural) close-fitting trousers of heavy denim for manual work or casual wear	(usually in the plural) a garment extending from the waist to the knee or ankle, covering each leg separately