Rant: Dirty diesels

[Bossxr8]

I keep hearing from the manufacturers that modern diesels are now very clean and are environmentally friendly, but what happens after they get a few thousand k's under their belts. I have noticed recently that a number of these so called clean modern diesels still belt out a nice big puff of black smoke under moderate acceleration. I'm not talking about 5 year old Landcruisers with 100,000kms on them but near new Audis, BMW's and Mercs. A diesel X5 that couldn't have been more than a year old belted out the black smoke as did a diesel A4 and an near new E class Merc. None of them would have been more than 2 years old yet the black smoke from the exhaust was clearly visible and you could smell it while driving behind them.

Do diesels only stay clean when they are new and get dirty with age or what.
Diesels are also supposed to pump out heaps more NoX emissions than petrol engines as well. Are diesel engines really clean or is it just BS from the manufacturers. I'd hate to see what they were like after 10 years and 200,000 kms. Anyone else noticed this?