Hmmmm. Well, OK. One thing that comes to mind is to get the right plugs for the engine. Different engines use different spark plugs. In fact, they can have different lengths that can cause severe problems if you put a long spark plug in an engine that requires a short one - the piston will contact the spark plug.
The other issue is the correct heat range of the spark plug. The same size spark plug comes in different heat ranges, typically to match the burning conditions in the cylinders based on things like compression ratio.
So, make sure you get the spark plugs not for a 280SL, but for the engine you have in there.
With the black smoke and black residue, you must be running rich on the fuel.
Is this a new-to-you car? Or is this a situation that has just started to happen with a car that has run well for a long time up to this point?