Black smoke - 420SEL

Yes, your car does have O2 sensor. If you haven't change it for a long time, give it a shot.

I'd suspect your timing chain may be the issue.

Another thought... pull out all your spark plug... see if any of them is fouled. If yes... then most likely the fuel distributor needs to be rebuilt.

Yes, it is a mixture problem for sure (much too rich)

Is this a model, which uses EHA (Electro Hydraulic Actuator - attached to the side of the fuel distributor)?.

This device fine adjusts the fuel pressure and thereby mixture according to feedback from the O2 sensor. When defective, it will create symptoms as described.

I had exactly this problem on a 1987 190E 2,3 (different engine but same injection system).

If your mechanic tried to adjust the mixture while EHA or O2 sensor was defective, it should be readjusted after correcting the problem.

The Air Flow Meter (big flap in intake) controls the fuel pressure according to the setting of mixture adjustment screw, and basically the engine will run with this alone (as it was the case until electronics were added, and as it is still the case until O2 sensor is warm). The fuel pressure regulator provides a stable pressure to this system, and a defective fuel pressure regulator may also cause wrong mixture.

The electronics consist of O2 sensor, ECU ("computer"), some other sensors (engine temperature etc.), and the EHA, which as said fine controls the mixture in a closed loop. Fault in any of these parts may cause the problem.

On alldatadiy.com you can find information on how to troubleshoot the loop.