Vehicles before 1996 will sometimes emit black exhaust smoke during the first start up of the day, this is normal as long as it clears up once warm. On vehicles built after 1996 , Black exhaust smoke is not normal. This is an indication that the engine is not burning the fuel efficiently and or the fuel injection system is trying to force to much fuel through the engine.
Possible causes range from a failed O2 Sensor, Injector Stuck open, or failing MAF Sensor. Some times a simple tune up is all that is needed.