Blackish central tail feathers separates this species from White-browed Robin-chat (C. heuglini), which has brown central tail feathers.
Be aware though, that rarely the central tail feathers on Ruppell's Robin-chat can appear dark brown on worn birds and/or in strong sunlight.
A hint of how this can look is seen on this picture (some brown color can be seen on the left side of central tail feathers)
English version