could be a few things that causes it, poor ventilation, water getting in from outside, not enough heating. my hallway in my other flat used to drip water until they put a vent on wall and a heater at bottom of stairs. can be solved quite easy when you know the cause.
Annoying thing is its only been decorated just over 12 months,and the other excuse he gave us is that they are outside walls,so basically he is saying that every house on the planet will have damp black walls,we all have outside walls,pillock. 
I know all about this one, as loads of our rental props have suffered this year and are covered in black mould on the the inside of external walls. Sometimes it is due to an insulation issue, sometimes ventilation.
The key is that the area of mould is usually the coldest part of the room (barring the window) and vapour condenses there-this vapour can come from anywhere in the house-not just the affected mouldy area; this is where the vapour goes TO and is not always the source, eg poorly maintained gas fires or an unventilated shower can be the source. If the condensed vapour is not removed, then mould grows.
My Parents bungalow has the same issue in one bedroom and after much investigation the problem was found to be a combination of part single brick wall, part failure of the DPC, part failure of the cavity wall insulation.
All of the cavity wall insulation has had to be removed, the DPC uprated and the single brick bit lined with insulation and plasterboard- they are doing it now.
One word of caution is that this mould can initiate or aggravate a number of respiratory problems, so must be rectified.
It can be rectified as per my Parents' place, but investigation as to the source of the vapour is key, and repair work can be costly and disruptive work.
If you are renting a council property insist that this is done on health grounds-they will take you more seriously.
Hope this helps
