When the car doesn't want to restart, disconnect the mass airfow sensor and try to start the engine. Does the engine start now? If it starts now the sensor is bad. Do not disconnect the sensor with the ignition on it will damage the sensor. If the above doesn't work try flooring the gas pedal while you crank the engine, it will turn the injectors off and allow any excess fuel to burn off the plugs. You might have a leaking injector that is causing the black smoke out the exhaust. The engine would start okay when cold because any excess fuel leaking by the injector would have evaporated off while the engine cooled off. If you can pressure check the fuel injection system it will show the leaking injector. If you don't have a pressure gauge, remove the injectors from the manifold leaving them connected to the fuel lines. Pressurize the fuel system and check for any dripping injectors. Fuel pump check valves that are bad generally result in long cranking times, but not black exhaust smoke. My bet is a leaky injector. Let me know what you find. John