“blue book”是什么意思？

Blue book完全没有任何下流的意思，之所以被人误传为“小黄书”是因为blue在作为形容词时，除了有蓝色/忧郁的意思，同时也有“淫秽、下流”的意思（showing or mentioning sexual activity in a way that offends many people）。
因而，黄色笑话可以说成“blue joke”或者“blue story”。当别人开黄腔时让你感到不舒服时就可以说：“Can you stop cracking blue jokes? I feel so bad.”（当然语气不是很礼貌）
牛津词典官网对“blue book”给出了四种解释：
▷ a book with a blue cover used by students for writing the answers to examination questions in
美国学生考试时用来答题的蓝皮答题本。
▷ a book that lists the prices that people should expect to pay for used cars
罗列了二手车交易价的蓝皮书。如著名的“凯利蓝皮书”（Kelly Bule Book, KBB），是北美二手车行业的标杆性定价杂志。
▷ an official report published by Parliament, usually from a government committee or a Royal Commission, published in a blue cover
官方发布的报告，通常为蓝色封皮。
▷ a book that gives details of people who have an important position in society
名人录
举个例子：
You pretty much just had to write your name on your blue book and you passed.
你差不多只需要在答题本上写上名字就通过考试了。