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.
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)
登入就可以看每一支怎麼連回核心畫面。