This type of dampness will only be on external walls or if there is a roof leak (on ceilings). It only appears because of an external defect to the property, such as missing pointing to the brickwork, cracked rendering or missing roof tiles. These defects allow water to pass from the outside to inside surfaces.
This is far more noticeable after rain and normally appears as a well-defined damp patch and looks and feels damp to touch.
Note – Black mould is rarely seen on areas of penetrating dampness, because the affected area is usually too wet and the dampness contains salt picked up when passing through the wall, which prevents black mould growth.