rpete,
With 5000 hours and excessive black smoke under load, I would expect that you need to have the injectors checked or overhauled.
Is the engine going through more than normal amounts of oil? This would be more of a bluish smoke anyway.
Have you checked your air filters? Has it lost power or just smoking black. I have changed injectors on a high hour engine before and it improved power and stopped the smoking. If you continue to run with a bad injector you could get cylinder wash and score a cylinder.
Dan