In politics,a dark-horse candidate is someone who is not well known to the public.

VOA: special.2009.07.12

It's dark. The other, dark, in the sense of gloomy, not good, bad; this is a hard time, a poor time, a wretched time, a miserable time.

因此称之为黑暗,另一个理由是,黑暗从悲观的角度来说就是不好的,坏的,这是一个艰难的时代,一个穷苦的时代,一个不幸的时代,一个悲惨的时代

耶鲁公开课 - 古希腊历史简介课程节选

Now, I've been saying kind things about Western civilization, but I would not want to deny that there is a dark side to the western experience and its way of life.

我刚刚一直说的都是西方文明光辉的一面,但我是不得不承认,西方文明,也有它黑暗的一面

耶鲁公开课 - 古希腊历史简介课程节选