The reason for the black smoke is that the air to fuel ratio is to rich. This can be caused by not enough clean air is getting to the engine (i.e the air filter is blocked) or that the Injectors are worn .You do not say how many miles the engine has done but if it is around the 100k mark then it may require that the injectors are serviced or replaced.

If you do a lot of town driving, it could be that the DPF (diesel particulate filter) is starting to get blocked or the EGR (exhaust gas recycling valve) is also starting to get blocked. Find a decent local garage and get them to carry out further checks.