directory vs blue book

directory and blue book both are nouns.

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

As nouns, blue book is a hyponym of directory; that is, blue book is a word with a more specific, narrower meaning than directory:

Other hyponyms of directory include phone book, phonebook, telephone book, telephone directory.

directory (noun)	blue book (noun)
an alphabetical list of names and addresses	a register of persons who are socially prominent
(computer science) a listing of the files stored in memory (usually on a hard disk)	a report published by the British government; bound in blue
	a blue booklet used in universities for writing examinations