It could be mould. Mould grows in atmospheres where there is a moisture content, with little ventilation. It likes areas behind cupboards and also lower walls. This is remedied with watered down bleach to kill (it's a living organism), then stain blocked, followed by decoration. It will return if ventilation is inadequate. Opening windows ajar helps. No wet clothes on radiators etc...
It is also a health hazard and must be gotten rid of. A decorator worth his salt will deal with this.
I hope this helps.