1996 Cadillac Deville black smoke, check emissions system w

Northstars use oil because the rings stick due to carbon buildup. Blue smoke would be an indication of burning oil, black might indicate excessive fuel. If you have no check engine light, you should not be worried. If you are concerned of the oil it is consuming, fill it to the full line on the stick, and monitor after 1000 miles, 2000 miles, etc. An engine with your kind of mileage would not be abnormal to be using a quart in 1500 miles or so, and this is not enough to cause noticeable smoke.