No I accept it's a straight unretouched negative. But if the floor was a little dusty, those really do appear to be footprints. It doesn't look like "a lot" of charcoal, and if the light was flat and the negative developed longer, then light dust would darken to charcoal. There doesn't even have to be that much of it.
Anyone know if dancers prefer dust on the floor, like mountain climbers chalk their hands, during a performance?