A blade (1) for a tail anti-torque device of a helicopter, said device comprising a ducted rotor (13). The blade (1) has an assembly of sections, a leading edge (2) and a trailing edge (3); a stacking line (4) of the profiles extending at a distance lying in the range 25% to 50% of the chord (C). The stacking line (4) presents a shape that is curved having successively from the root (5) to the end (6) of the blade a back sweep, a forward sweep, and a last back sweep; and a chord (C) that becomes larger going towards the end (6) of the blade (1) over at least the end profile (1a).