Dry with carbon build up, and black smoke ususlly indicates running to rich on fuel. If your plugs show carbon, plus oily residue, you're getting oil past the rings, or valve guides.
The standard plug was a Champion D15Y or equivelent. It was common to step up to a hotter plug to get by for a while, to a D21Y until the engine could be overhauled, or re-ringed at the very least. If left go long enough, and you tear the engine down, when pulling the pistons, you'll probably notice a similar buildup in, and around the oil rings, which only allows more oil to pass by them.
