under cover of night; under cover of darkness
Fixed expression meaning taking advantage of darkness to act secretly or escape notice.
The culprit escaped under cover of darkness.
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'.