Improper air adjustment

All fires need air!  If your burner is not getting enough air it will emit black smoke.  This is not the only reason for black smoke but it is the best place to start diagnosing the problem.

All burners have an air adjustment.  You can set the amount of air going into the burner chamber by finding the adjustment and moving it.  Check and see what happens to the exhaust from the burner as you adjust the air.

If, by adding more air, the exhaust does not clear up, you have another problem.  Either too much fuel, wrong fuel or faulty igniters can case black smoke as well so check these out.

Note- professional service people will have gauges that detect smoke level and flue temperature.  If you continue to have problems it's a good idea to seek help.