The black redstart is 13–14.5 cm (5.1–5.7 in) in length and 12–20 g (0.42–0.71 oz) in weight, similar to the common redstart. The adult male is overall dark grey to black on the upperparts and with a black breast; the lower rump and tail are orange-red, with the two central tail feathers dark red-brown. The belly and undertail are either blackish-grey (western subspecies; see Taxonomy and systematics, above) or orange-red (eastern subspecies); the wings are blackish-grey with pale fringes on the secondaries forming a whitish panel (western subspecies) or all blackish (eastern subspecies). The female is grey (western subspecies) to grey-brown (eastern subspecies) overall except for the orange-red lower rump and tail, greyer than the common redstart; at any age the grey axillaries and underwing coverts are also distinctive (in the common redstart these are buff to orange-red). One-year-old males are similar to females but blacker; the whitish wing panel of the western subspecies does not develop until the second year.