藍皮＝官方認可的數字
這一組共用「藍封面代表可靠」這層意思。書裡已經印好內容，你去查，不是去寫。
noun · C1
A book of official facts and figures, such as statistics, records or lists of names, published by a government office or by a trade group.
藍皮書；官方資料彙編
核心畫面
Picture a book with a blue cover sitting on a desk, and everyone in the room agreeing that what is in it settles the question. That is the older picture: blue cover, official contents. A second, much plainer picture sits next to it, where the blue cover is only a blue cover and the pages are empty, waiting for you to write.
可以想成桌上放著一本藍色封面的書，而房間裡的人都同意：裡面寫的就算數。這是比較老的那個畫面，藍封面代表官方、代表可靠…更多收起
可以想成桌上放著一本藍色封面的書，而房間裡的人都同意：裡面寫的就算數。這是比較老的那個畫面，藍封面代表官方、代表可靠。旁邊還有第二個素得多的畫面：封面一樣是藍的，但裡面是空白的，等著你自己寫。這個字的幾個意思，就分別站在這兩個畫面下面。From the word’s history
The source dates the name to the fifteenth century, when large books covered in blue velvet were used for record-keeping by the Parliament of the United Kingdom.
來源說這個叫法可以追到十五世紀：當時英國國會用來做紀錄的大本子，外面包著藍色絲絨。名字就這樣留下來了。這也說明了為什麼「藍皮書」直到今天還帶著「官方、可靠」的味道，連中古車的價目書都借用了這層意思。
Wiktionary entry for blue book, etymology note (kaikki snapshot kaikki:3f6f69a190036ae4:2186314663:1590)
Two branches
藍皮＝空白的本子
這一組跟權威沒有關係。藍色就只是封面的顏色，裡面還沒有任何內容。
noun · C1
A thin book with a blue cover and empty lined pages, used in American colleges for writing exam answers by hand.
（美國大學）考試作答本