Dampness can be caused by leaks from water and waste pipes, especially in bathrooms and kitchens. This can affect walls and ceilings both inside and outside of the home. It causes a damp patch which looks and feels wet to the touch, but it stays damp whatever the weather is like outside.
Once the leak has been fixed, the damp patch should fade and dry out over time.
Black mould will rarely be seen on this type of dampness because the area is too wet and the chemicals in a waste water leak will prevent mould growth.