Black soot in or on a tailpipe means that the air-fuel mixture that drives the engine is not burning completely. An overly rich fuel mixture sometimes leads to black soot in the tailpipe.
A rich fuel mixture is a sign that the air filter is clogged or something is wrong with the fuel-intake system. For instance, the fuel control sensor may be faulty, the fuel injector may be leaking or clogged, the fuel pressure may be too high or the pressure regulator may be leaking. If a car is producing black soot, it is best to have it checked by a mechanic for the right diagnosis. However, it is normal for car engines to produce a small amount of soot since most of them are not perfectly efficient in burning fuel.