blue book vs leaflet

blue book and leaflet both are nouns.

Word	Noun	Adjective	Verb	Adverb
blue book	Yes	No	No	No
leaflet	Yes	No	No	No

As nouns, leaflet is a hypernym of blue book; that is, leaflet is a word with a broader meaning than blue book:

Other hypernyms of blue book include booklet, brochure, folder, pamphlet.

blue book (noun)	leaflet (noun)
a register of persons who are socially prominent	a thin triangular flap of a heart valve
a report published by the British government; bound in blue	a small book usually having a paper cover
a blue booklet used in universities for writing examinations	part of a compound leaf