Paint the inside of the mine black? After all, it IS a hole in a coal seam, so the walls, at least are coal, and the floor is covered with coal dust. The roof may not be solid black, depending on the type of rock, and on how much dust has settled on it. No soot, as no combustion engines inside, so if it's a light colored rock, and the coal separates cleanly, it may not be that dark, at least at the entrance. It will be dark a little ways inside...
