I think it is soot. Thick black soot. The best way I've found to get it off at the moment is to apply a thick paste of washing powder, leave a few hours, then wash off, taking quite a bit of the black stuff with it.
Under that there's a thin rust-coloured layer onto the bare stone. That might...
The stone lintel above my fireplace is covered in black paint that smells sooty and smudges on your fingers. I want to re-paint it neatly in black (it was covered up by four layers of wallpaper and woodchip until recently).
Have tried HomeStrip paint stripper, but that didn't do much better...