Black mold can be found on and behind walls because the environment is perfect for black mold to grow in. Black mold prefers dark and damp places that usually can stay within a certain temperature range and not get too cold. Moisture can get trapped in walls simply from using the shower or taking a bath, or even boiling water on the stove. Doing these things releases humidity into the air, which can then get trapped inside walls giving mold the perfect humidity to survive.
Another reason black mold may grow so well behind walls is because most water pipes that are found in your home are behind walls, thus giving more moisture for mold to grow. These pipes may leak from time to time, as well. If a pipe breaks, and there is flooding behind a wall, it will be extremely hard to clean up all the water and moisture, giving black mold more dampness to thrive on and continue to grow. Mold on walls will usually cause a yellow stain, or if the wall is painted, it will cause the paint to start peeling. Mold growing on walls is a lot easier to see than mold growing in dark places.