Meaning:
“灯下” means “under the light,” and “黑” means “dark.” Translated as “under the light but in the dark,” this term is used as a metaphor to refer to the scenario where people tend to neglect the most obvious things. Sometimes, it is used to refer to the situation where officers working with crime-fighting authorities have committed crimes themselves. Some also use it to warn people against enemies lurking nearby or posing as allies.
Example:
A: 听说你姐要结婚了,谁运气那么好?
Tīngshuō nǐjiě yào jiéhūn le,shuí yùnqì nàme hǎo?
I’ve heard that your elder sister is getting married. Who’s the lucky guy?
B: 是她高中同学,多年的好朋友,忽然有一天发现他俩情投意合挺合适。
Shì tā gāozhōng tóngxué,duōnián de hǎopéngyou,hūrán yǒu yītiān fāxiàn tāliǎ qíngtóu yìhé tǐng héshì。
He’s her high school classmate, a friend for many years. One day it suddenly occurred to them that they are kindred souls and could be soul mates.
A: 真是典型的灯下黑。
Zhēnshì diǎnxíng de dēngxiàhēi。
That was a typical case of not seeing the obvious.