Their dark humor never failed to astound him.是什么意思？

Their dark humor never failed to astound him.

他们的黑色幽默总能让他咋舌。

their

pron. 他（她，它）们的，在提及性别不详的人时，用以代替 him 或 her；

例句
Their permissiveness toward their children reflects the wild abandon of their own lives.
他们对孩子的放任折射出他们自身生活的放浪形骸。

dark

adj. 黑暗的，乌黑的，忧郁的，神秘的；n. 黑暗，暗色，暗处；

例句
In the dark my sense of hearing becomes so acute.
黑暗中我的听觉变得异常灵敏。

humor

n. 幽默，心情，诙谐；vt. 迎合，牵就，顺应；

例句
Shakespeare has accustomed us to a mixture of humor and tragedy in the same play.
莎士比亚让我们习见了同一场戏目中的悲喜交错。

never

adv. 从不，从来没有，一点也不，决不，<口>不会…吧，没有，不曾；

例句
The public never had faith in his ability to handle the job...
公众从来不相信他有能力胜任这一职位。

failed

adj. 失败了的，不成功的；v. 在…中失败( fail的过去式和过去分词 )，衰退，衰弱，破产；

例句
They recently failed to negotiate a mutually acceptable new contract.
他们最近未能谈成一个双方都满意的新合同。

to

prep. 向，朝着，到，关于，属于；adv. 朝一个方向的，到某种状态，关闭；

例句
I suggest you leave it to an expert...
我建议你把它留给专家来解决。

astound

vt. 使震惊，使大吃一惊；

例句
Their dark humor never failed to astound him...
他们的黑色幽默总能让他咋舌。

him

pron. (he的宾格)他；

例句
They said goodbye to him as he got aboard the train at Union Station.
当他在联合火车站登上火车之际，他们向他道别。