Metaphors for when it is really dark outside include "as dark as night," "like a black hole," "as pitch black as coal," "like the depths of the ocean," and "as dark as the inside of a cave." These metaphors convey a sense of complete darkness and absence of light.

A dark period of time is unpleasant or frightening. Once again there's talk of very dark days ahead. This was the darkest period of the war.