1990 Buick Lesabre

I've had this car for 4 years and it just now has started to act up. It is pushing out black smoke, has a missfire in cylinder 3, 5, and 6. It has been doing this off and on for 2 weeks now and getting progressivly worse. Ecm has been changed and the coil pac too, reacently did a fuel pressure check and it is fine running normal but slightly low (guessing because of ths problem). I was told it could be the cat convt. But now with the black smoke I have no idea.