What are some ways of dealing with black mold that grows between the walls of a house?

The best way to treat black mold between walls is to call a professional, as they will be able to treat all the mold, even where it is not easily visible or accessible. You can try to treat it yourself by using mold remover spray and then repairing any leaks that may be allowing water into the space. Just note that if the mold isn't completely eradicated, and the leaks/moisture eliminated, the mold can return and spread again.