[Superman]

What Asphaltgambler said. Today's building methods involve wrapping walls with plastic. This can be done so well that the wall is basically sealed hermetically. Perhaps that's fine if moisture never gets in, but in the real world moisture is in there. Walls need to have some ventilation.

Yes, bleach kills mildew (which is black) immediately right in front of your eyes. Even diluted with plenty of water, bleach kills the poop out of mildew. Again, I'd suggest ventilating the walls somehow, and also finding out how the moisture is getting in.