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。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, "blue book"
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