to grow dark, and so to go about darkened and gloomy in mourning
The core picture is a loss of light: something that should be bright turns dark. The word is used for the sun, moon, and stars going dark, for the sky turning black with storm clouds, and even for a stream running dark and muddy.
From that literal darkening comes its most striking use. A grieving person in ancient Israel looked the part: dusty, unwashed, and dressed in black. So the same verb describes someone who "goes about darkened" with grief. When a psalmist says he goes mourning all day long, the word paints his whole gloomy, downcast appearance, not just his inner sadness.
It sits close to the ordinary word for darkness (חשׁך) but adds the sense of something turning dark or dirty; and close to the ordinary verb for mourning (אבל), except that where that word names the grief and its rites, this one paints the mourner's darkened look.