[b]Barry:

If we are thinking about the same "Pot Black," also known as "Stove Polish," it doesn't adhere well, it's easily wiped off. This means the enamel over it would not be attached any better than the blacking under, and chip off easily.

Additionally, when "Stoving" an air cooled engine you need to keep the paint layer as thin as possible - when it gets thick it starts to be an insulation and you lose the advantage of the dark color's better radiation.

Bill.