The Black Redstart is a small passerine bird. It is 15 cm in length and 20 gm 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 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.