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Diesel engines are capable of air fuel mixtures of 100:1 vs gasolines 14.7:1 A gas engine running that lean would melt pistons. The black smoke is soot from overfueling. A well tuned diesel with a particulate trap emits almost no soot. The reason the truck going uphill in front of you is belching it is because its been on the road a million miles since it's last tune up.

Diesels also emit higher Oxides of Nitrogen (NoX) emissions because of increased combustion temps, but technology is all ready working on that. Hydrocarbon emissions on a diesel engine are much lower.

NEWER Diesels really do burn cleaner, but pedal to the metal type driving won't help at all(actually causes more "black stuff")

driving normal/easy you shouldn't notice any black smoke

Diesels also take a few minutes to warm up, and you may notice some black smoke it normal.

(the black is actually soot not caught by the particulate filter) For the most part 18 wheeler trucks don't have filters (newer ones do)

gasoline engines also have some smoke but its less noticeable

There are also "cleaner low sulfer diesel fuel" you can buy that helps on the pollution and visible smoke but its for mostly newer diesels

diesel engines do burn cleaner if you use sunflower oil i use old oil from burger shop or buy it at supermarket when the diesel engine was madeit was to run on peanut oil i run my landrover on old chip oil all you get is the smell of fish and chips