Black smoke is caused by an over-rich fuel condition. Or in the world of performance diesels "not enough air flow"
The thing about diesels is, unlike gasoline engines where it's recomended to run them slightly rich to be safe and avoid detonation. Diesels prefer to run as lean as possible. The more air you can get through the engine the more efficient it will be, and the cooler it will run. The only down side to that is, the lower the tempurature of the exhaust the less energy is present to spool the turbo. This has a huge impact on 6.0 PSD.
Several 6.0 PSD owners have noticed that after improving air flow by way of aftermarket exhaust or intake. Acceleration actually drops. On a good note mileage usually improves a little. The correction for this is adding more fuel by manipulating the amount of fuel the injectors squirt into the engine (obiviously this will bring the mileage back down, but only when getting on the loud pedal)
To answer your question GJP1 this guy probably has a aftermarket tune on his injection system, which is adding more fuel. Possibly too much. Alot of tuners in on the market have several power settings. Most discorage using the higer power levels for towing due to the chance of damaging the engine. This Dodge guy probably ignored one of those warnings and is on his way to over heating his engine. But if he is driving a Dodge then obviously he isn't the wisest guy on the road :p