What should I do if the wall is damp, moldy and black?

First of all, let me help you analyze the reason: the weather in the north is very dry in winter, and the inner wall will not be wet because of the external humidity. So your situation must be caused by the large temperature difference between indoor and outdoor in winter, which leads to condensation on the inner wall. This reason is mostly because the external wall insulation is not done well or not done at all. I met this situation in Beijing Huilongguan Community. The developer did not do external wall insulation, which caused dozens of residents in the community to get wet and leak in winter. The best way to solve the blackening phenomenon is to contact and coordinate with the developer to do a good job in external wall insulation. If you can't do it for a long time because of the delivery time, you can only take some internal decoration measures to make up for it, such as repainting the wall and thickening it. In addition, some wall building materials with good moisture absorption function can be used. For example, I recommended a Japanese paint called "Breathing Room" to some owners in Huilongguan. This paint has good moisture absorption performance and can adjust humidity according to the environment. You can take corresponding measures according to your own situation.