It might just need a good service. Generally speaking, black smoke is produced when there is plenty of fuel being injected, but not enough air for an efficient burn of the fuel. That said, there will always be a trace of black smoke when the engine is fueling correctly - it's better to have a little more fuel than necessary than not enough. Have a look at your air filter element and if it is filthy - replace it. Also check that there are no leaks along your air intake hoses (particularly between the turbo outlet and the inlet manifold). Remove all your air intake hoses and check for signs of delimitation. As hoses get older, the internal rubber / silicone laminate breaks away from the outer supporting laminates. As the engine draws air in, any saggy / flappy bits along the pipe work are sucked flat and the engine is starved of air which results in plumes of black smoke and a distinct lack of power. A collapsed or ruptured intercooler will also have similar effects.
Alternatively, as suggested by Storm, it may just have had the injection pump played with to get a little extra power. If you take a look at the very top most part of the injection pump (the cover held down with four bolts), is there a tamper proof cap in the centre, or can you see a circular recess with a hex-bolt and a lock nut? Also, do the four bolts that hold the top cover down look to have been removed in the past? If you suspect the pump has been tweaked, DO NOT just try to adjust it back without knowing what you're doing or you will likely bugger it up completely. Another possibility is that your injectors need new nozzles and the pop-pressures resetting but they are usually good for a couple of hundred-thousand miles before they need any attention, so start with the simple things!
Mine has a brand new injection pump, new injectors, new turbo and all new hoses and it smokes like a bitch when I deck it - it shifts a fair bit too but that's fuel pump tweaking for you

Start simple - replace all your filters, give it some fresh 10-40 oil and fill your new fuel filter with injector cleaner before you screw it on.
-Tom