tiān​hēi

nightfall

天黑 tiān​hēi
nightfall noun
the time of day when the sky turns dark; dusk

We must reach our destination before dark.

get dark noun
to become dark after sunset

It's getting dark so fast; I can't even see the road anymore.

Frequency

Written text
4.72 per million

Sources

Definitions
CC-CEDICT
Frequency data
BCC Corpus (BLCU)