When did the trouble start - When troubleshooting a problem it's always a good idea to start at the beginning.
If you can identifiy what has changed you can often zero in on what has failed or is not working right.
If it has been a gradual change over time, it could be that the engine is old and ready for overhaul
. Lot's of stuff between a and b.
In simple terms black smoke will usually indicate the engine is under excessive load for the speed. i.e. the throttle is open to a point where there is too much fuel
being dumped in for the air flow (rpm) and unburned fuel is coming out the exhaust
. With a lower load the engine would speed up and "balance" the fuel air mixture. Hnece the question does it smoke in neutral at any RPM and does it make "full" rpm?
Blocked inlet, increased resistance in the drive train - shaft/prop bearings, fouling, too high a pitch
etc - are all candidates.
A failed turbo could possibly do it but that would be low down my list of things to suspect.
Is there a reason you suspect the turbo?