We bought a modern dehumidier and it takes out 3-4 times the amount of water than the previous one (also has a nice clothes drying feature) often working when the other extracted nothing.
A bit of black staining spots ( very dark green maybe) on the wall which is also wipeable is likely to be condesation.
If you have a damp problem you are more likely to see a stain or discolouration..
Would you mind revealing what you bought? Asking for a friend.