I would suspect the coolant temp sensor for the fuel injection. If it is telling the computer the engine is cold the computer will run a rich mixture causing black smoke and rough running once the car is up to operating temp. It usually has a blue top and brass body with a two pronged plug and is screwed into the intake manifold near the distributer or upper rad hose outlet. if I recall. You can check resistance across the two prongs with a ohm meter and compare your reading to the specs listed in the shop manual. They are cheap and easy to replace.