賽馬場上的那匹馬
這是字面那一格：資料空白的，真的是一匹馬。
noun · C1
In horse racing, a horse that people know little or nothing about, so no one can say how fast it is or how well it will run.
底細不明的賽馬
核心畫面
Picture the moment before a race, with everyone reading the card. For most of the horses there are numbers to read: past , times, places. For one horse the column is nearly empty. Nobody has watched it run. That empty column is the whole idea. Dark here does not mean bad, and it does not really mean the colour of the coat — it means you cannot see in. Everything the expression does later comes out of that one blank space where information should be.
可以想成開賽前，大家低頭在看成績表的那一刻。大部分的馬都有資料可讀：跑過哪幾場、時間、名次。可是有一匹馬，那一欄幾乎…更多收起
可以想成開賽前，大家低頭在看成績表的那一刻。大部分的馬都有資料可讀：跑過哪幾場、時間、名次。可是有一匹馬，那一欄幾乎是空的。沒有人看過牠跑。那個空欄位就是整個詞的意思。這裡的 dark 不是「差」，也不真的在講毛色，而是「看不進去」。後面所有的用法，都是從那塊本該有資料、卻空著的地方長出來的。From the word’s history
Wiktionary records the expression as originally an allusion to an unknown horse with a dark coat winning a race, as used in Benjamin Disraeli's 1831 novel The Young Duke.
Wiktionary 記載：這個說法本來是在借用賽馬的畫面——一匹毛色深、沒人認識的馬贏了比賽；最早見於 Benjamin Disraeli 一八三一年的小說 The Young Duke。
Wiktionary entry 'dark horse' (etymology_text; kaikki:3f6f69a190036ae4:1216910170:21634)
Three branches
競賽裡的黑馬
空白的資料欄從馬身上換到人身上，場地也從跑道換成比賽、選舉、比稿。結果一出來，大家才驚訝。
noun · B2
Someone who is not well known, and who few people expect much from, but who then does surprisingly well and may win — used about a competitor in a contest, and about a candidate in an election who comes forward when nobody had been talking about them.
黑馬
身邊那個深藏不露的人
這裡沒有比賽了。資料之所以空白，是因為這個人自己不講。
noun · B2
Someone who says very little about themselves, so other people are surprised when they find out what this person can do or has already done. Mainly British English.
深藏不露的人