Meaning of “darken” - Learner’s Dictionary

darken

verb [ I, T ] uk us /ˈdɑːkən/
DARK

to become dark or make something dark:

the darkening sky
a darkened room
MOOD

If someone's mood darkens, or if something darkens it, they suddenly feel less happy.

(Definition of “darken” from the Cambridge Learner’s Dictionary © Cambridge University Press)