Left sun radical (Kangxi #72) — the four-stroke pictograph of the sun with a central stroke for sunspots. Anchors
暗 in the time-of-day-and-light family alongside
明 (bright),
晚 (late),
早 (early),
暮 (dusk),
晴 (clear weather). Tells the reader:
暗 is fundamentally about light — specifically, its absence.