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Measuring 4.25-8.5mm, P. nigricans is black with a pale prothorax. Dorsally, the pronotum is often rosy and bears a wide median black vitta (stripe) and entirely black borders. In some southern U.S. localities, specimens of one or both sexes have been found with reduced elytra (Green 1961, Lloyd 1999). The species can be distinguished from other Alberta fireflies by characteristics of the antennae and pronotum. The first antennal segment is shorter than the third, providing a relatively easy way to distinguish P. nigricans from the similarly coloured Ellychnia species. The antennae of P. nigricans are flattened. This species can be further distinguished from other fireflies in Alberta by having the entire pronotal border outlined by an even black band, and by the lack of translucent windows in the pronotum. Propyga nigricans has 10.4-11.5mm long, campodeiform larvae. The body is parallel sided, with the last 3 or 4 abdominal segments tapering to a very small 10th (last) segment which houses the larval hold-fast organ (like a posterior foot). The larval body is dorsoventrally flattened to a limited degree, and has a dark brown colour with paler edges. Its head is slightly retracted into the thorax. Pupae of P. nigricans are 8.8-9.1mm (Archangelsky 2001).

In Alberta, specimens of P. nigricans have been collected in the southeast (Medicine Hat and the Cypress Hills) and Edmonton. Its Canadian range extends from B.C. and the Northwest Territories to New Brunswick (Bousquet 1991). The species ranges throughout most of the United States, except the southeastern portion, and is quite common from the Rocky Mountains westward (Green 1961).