Condensation, damp and the difference between them
Black spotting in a corner gets treated as one problem with one answer, usually a bottle of mould spray. In practice there are three separate causes behind it, and the spray only helps with one of them.
Condensation
Warm air holds moisture. When it meets a cold surface it cannot hold it any longer and the water appears on that surface. This is why it shows up on windows, on outside corners of rooms, and behind wardrobes pushed against an external wall.
The pattern gives it away: worst in winter, worst in the morning, spread across a surface rather than concentrated in a spot, and often in a room where washing dries or where showers are taken. Wiping it away each day treats the symptom.
What helps is reducing the moisture going into the air and letting it out. Cover pans while cooking, run the extractor fan during a shower and for a while afterwards, dry washing outside where possible, and leave a gap behind furniture on cold walls.
A leak
A leak is concentrated rather than general. One patch, usually with a clear edge, sometimes a tide mark, and it does not follow the seasons. It may be worse after heavy rain, or worse when a particular bathroom is used, which is a useful clue.
Trace it upward and outward. A ceiling stain rarely sits directly under its source, because water runs along a joist before it drops. Nothing about a leak improves with ventilation, so if better airflow makes no difference at all, stop treating it as condensation.
Rising damp
Rising damp sits low on a wall, generally no more than a metre from the floor, with a fairly level top edge and sometimes a salty deposit along it. It is far less common than the other two and is often diagnosed wrongly.
Before assuming it, check the obvious outside causes: ground level built up above the damp proof course, a blocked air brick, a path sloping towards the house, or a downpipe discharging against the wall.
Treat the cause, then the surface
Cleaning mould off a wall without removing the moisture feeding it gives a few weeks of improvement and no more. Deal with the water first, let the area dry fully, then make good the surface.
Where mould covers a large area, keeps returning after the cause has been dealt with, or someone in the house has a breathing condition, it is worth getting assessed rather than worked on repeatedly.