Picture the belts hanging on a rack in a training hall, running from white at one end to black at the other. The colour you tie on says how far you have come, and black is the last one in the line. Everything this word does can be traced back to that one strip of cloth: the level itself, the person allowed to wear it, and, said with a smile, anyone who is that good at anything at all.
可以想成道館牆上掛著一排腰帶，顏色從白一路排到黑。你腰上綁哪一條，就代表你練到哪裡。黑色排在這條線的最後。這個字的幾種用法，就像都從這條布長出來的：它可以指那個等級本身，也可以指有資格綁上它的人，還可以半開玩笑地借去形容任何領域的高手。
From the word’s history
English took the term straight from Japanese 黒帯 (kuroobi), literally black + belt, along with the belt system of the martial arts themselves, so the words and the object match exactly.
英文的 black belt 是直接照日文「黒帯（くろおび）」翻過來的：黒＝black，帯＝belt。這個說法是跟著日本武術的腰帶制度一起進入英語的，所以字面和那條實物完全對得上。
Wiktionary, "black belt" (etymology 1)
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