under cover of night; under cover of darkness
Fixed expression meaning taking advantage of darkness to act secretly or escape notice.
犯人は闇に紛れて逃げ去った。
The culprit escaped under cover of darkness.
闇に紛れて忍び込む。
Sneak in under cover of night.
Composed of 闇 (darkness), に (particle), and 紛れて (te-form of 紛れる, 'to be lost in' or 'to be indistinguishable from'), literally 'being lost in the darkness'.