The Black Redstart is 13–14.5 cm in length and 12–20 g 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 or orange-red; the wings are blackish-grey with pale fringes on the secondaries forming a whitish panel or all blackish.
The female is grey to grey-brown 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. 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.