Hi, most probably you have a liking valve seal in one of the cylinders, (excuse my spelling cause I´m mexican
) this will cause some oil to go inside of the cylinder when the car is off, then by the time you start it, it will burn that oil out (that explains the black smoke) and it will stop when all the oil is burned (1 or 2 min, aprox). Then the idle will be fine. It´s not a big problem, you could fix it yourself if you get a valve spring opressor to get the seals out. Change them all at once.