Why is my exhaust gas black

Dark smoke is usually either from too much gasoline or diesel being introduced or from burning oil.

If it's also using oil at a higher rate, it's probably at least partially oil being burned.

If it's oil, it's likely leaking valve seals or something similar that allows oil to seep into the combustion chamber. If so, it will only smoke after a significant amount of oil leaks into the chamber.

Oil burning will eventually clog the spark plugs on gasoline engines. Check for black crystalline or oily residue.

If it happens every time you start it, it's probably a fuel injector or carburetor or sensor problem.

If it changes with ambient temperature, it's probably a sensor telling the engine it's very cold and to enrich the mixture.