Check the air-filter first...This is often overlooked ,even by dealers.
The fuel pressure sensor is also a common culprit.
You say that after replacing the MAF car was good for a while,before you got the black smoke again!Dont ignore the fact that even the new MAF could have failed.
Have you checked the engine breather?
Have you checked if EGR is stuck closed...?Pull out the vacuum pipe from egr valve and scew in a 6mm screw on pipe and test car.
Also check the turbo VNT (N75) valves as these have a wire filter that gets blocked.
IF after all this and after checking all the pipe connections ,as Lawrence suggested,car is still the same,then it can only be the turbo and intermitent sticking vanes.