sounds like one or more bad injectors to me. Take them out and
1) buy rebuilts
2) buy new ones
3) get them rebuilt.
The black smoke is the usual giveaway for bad injectors. If you rev up the engine once warmed up you will most likely see tons of black smoke belching out the exhaust but disappears or becomes significantly less at normal revs up to 3000RPM.
I maintain 3 VW diesels in my family and have seen this several times. I am lucky enough to have an injector rebuilding shop nearby that charges $38US each for VW injector rebuilding with a 1 year warranty. 200,000 miles on injectors is about right depending upon fuel. Newer diesel fuels here in Canada don't use sulphur anymore for lubricating the pump and injectors. If you do not use an additive to lubricate them they will not last as long. If you can still get diesel with sulphur where you are you will get longer life out of pumps and injectors. One trick an old diesel mechanic taught me is to dump a quart of ATF into the tank once a month - it cleans and provides lubrication. I had a high miler diesel and tried it - it worked as good as diesel conditioners at a fraction of the cost.