Its genus name, Astrapia, means "flash of lightning", referring to the brilliant iridescent plumage present in some form on all astapias.

The males of this species reach around 84 cm (33 inches) in length, and a considerable portion of its length comes from the long, 47 cm (18 inches), glossy, black central tail feathers, which almost compete with the exaggerated, white tail feathers of the male Ribbon-tailed astrapias.

The upper head of the male is an iridescent greenish-blue, shading to indigo blue in the back; the chin to the upper breast section is an iridescent aqua to greenish-blue, though the throat area is usually a dark, shaded tint depending on the angle of view.

The most distinctive feature, the extremely exaggerated central tail plumes, are wholly black with a purple gloss, and white rachides.

The feet are grey-blackish, the bill is blackish, the mouth is pale green, and the eyes are dark brown.

This form of astrapia has extremely long central rectrices that have variable amounts of black and white, and are sometimes seen in a mysterious, spiky shape near the end of the tail.

Stephanie's astrapia, like a majority of its relatives, are shown to be predominantly frugivorous, particularly favoring members of the genus Heptapleurum.