Plants loosely cespitose, mat-forming; rhizomes short, stout. Culms 5–30 cm. Leaves flat to the tip, (1.5–)2–4 mm wide. Pistillate scales reddish brown to black, lanceolate, as broad and as long as or slightly longer than perigynia, margins not hyaline or scarious, apex acute to acuminate. Perigynia ascending to spreading and deflexed at maturity, 3.8–4.1(–5) × 1–1.2 mm; beak dark brown to black, frequently as long as body of perigynium. Stigmas 3. 2n = 72.

Fruiting Jul–Aug. Moist meadows, mossy heaths, wet gravels of streams and snowbeds; 90–3400 m; Alta., B.C.; Alaska, Calif., Colo., Mont., Oreg., Utah, Wash., Wyo.

The occurrence of Carex nigricans in Russia, although expected, is unconfirmed by specimens, according to T. V. Egorova (1999).