blue book vs written report

blue book and written report both are nouns.

Word	Noun	Adjective	Verb	Adverb
blue book	Yes	No	No	No
written report	Yes	No	No	No

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

blue book: a report published by the British government; bound in blue
written report: a written document describing the findings of some individual or group

Other hypernyms of blue book include report, study.

blue book (noun)	written report (noun)
a register of persons who are socially prominent	a written document describing the findings of some individual or group
a report published by the British government; bound in blue
a blue booklet used in universities for writing examinations