Black smoke from exhaust pls help

I just bought a 1996 accent for my son.It is his first car.It only has 78,000 klms on it.Test drove it and all seemed well. When I got home I noticed that the exhaust was spitting out alot of water and it was also putting out black smoke.The idle is rough.I think this car has been sitting around for a while.The water coming from the exhaust has now slowed down to almost normal. The oil on the dip stick looks normal and is not milky and the rad fluid is topped off. What is my problem here? Tune up? I seems fine on the highway.The engine sounds fine at higher speeds.No check engine light on.

Congrats to your son for getting a car  Black can be either black smoke (unburnt fuel) or carbon buildup. Blue smoke is when your car is burning oil, and white smoke can be a coolant leak.
 
There are a number of things that can cause unburnt fuel, too many to pinpoint. Carbon coming out of the exhaust can show if the previous owner never really blew the cat out (got the engine rpm high enough to make air flow fast through the catalytic converter).
 
If it was sitting awhile, you need to change the oil and oil filter. Check the air filter and in-cabin air filter. Check the other fluids - tranny, power steering. For cheap insurance, you could even change the plugs and plug wires. It is much cheaper for you to change/diagnose this stuff than let a shop do it. If this stuff doesn't fix it, you may want to take it to a shop. It could cost more in the long run if you don't.

heated o2 sensor replacement fixed it.

Oh I forgot to note that the previous owner pulled the MIL lamp, hiding the warning and alerting me to the O2 sensor code.