That car runs an EFI system that may be running rich. Smell your oil for gas and check for oil on the EFI cams and points under the ignition breaker plate. Oil on the contacts acts as if the points fire for a longer period of time and leaves the injectors open longer. Result is black smoke. Don't know if that's the problem but they all need cleaning sooner or later and yours seems to be creaping at the correct rate.