It is likely to just be a mould --- Americans call it toxic mould and its not good if you suffer from any respiratory problems, may also be referred to by some as mildew, but generally whatever, proprietary mould killer or common bleach etc plus improved ventilation and increased heating to the room will usually eliminate it.
Is it near a source of incoming cold air like a wall vent?
Bedrooms do generate lots of moisture from humans breathing overnight and so bedrooms are as susceptible as bathrooms and kitchens.
Often develops on cold North East corners of a building or where there may be a 'cold thermal bridge', although a new house should have such things as thermal bridges designed out prior to construction.
http://www.nhbc.co.uk/NHBCPublicati.../HomeownerDocuments/filedownload,31929,en.pdf