I suppose its the same principle as waxing a terracotta tiled floor with colored wax. I have tried to polish up a slate hearth before and it just didn't happen. I wish I had have thought of this. I would only be concerned with how the shoe polish holds up to the heat.
I've seen it work nicely on expensive and very old William and Mary furniture from the 16 or 1700s. Covered up dings beautifully before the auction! ( no I didn't say it was me!)
You must also remember melting the stuff to bull yer boots so personally I wouldn't be sticking it on a fireplace lol