I'd be worried, if there was black smoke present all the time, rather than when under load. And I'm presuming that the engine isn't using oil?
You might want to try using a Diesel fuel additive (I've tried Miller Oils) and give the car a good run (extending it through the gears a bit - when its warmed up & where legal of course ). This should help clear things out as far as is possible, and possibly give a bit more power & mpg as well.
Well, kind of - what's called an "Italian tune-up", is to take a car which is normally driven very gently and/or in town out on an extended run whereby it is driven very hard through the gears. Using a fuel system/injector additive would make this type of flushing of the engine a bit more effective (and probably better for it in the longer term too).