I don't like putting a blackened pot back in my rucksack
I try and keep my pots shiny. I have to say though cooking over a good pile of embers is a lot more efficient than putting a pot on the fire too early and pretty much avoids blackening.
I would wonder about the insulating effect of a layer of soot and its bound to be a bad conductor of heat. Black is also a better radiator of heat so black pot sides will be loosing you heat.
But then witches are always standing around a black cauldron!??