Whether it's in a corner of the basement, on the closet ceiling or on a stack of loose sheets in the garage, black mold on drywall always indicates moisture, and it's a bad sign. Drywall is porous, and if you can see mold on the surface, there's a good chance it's also growing in places you can't see. Active scrubbing is needed to remove it, and moisture control prevents it from growing back.
Before you attempt to remove mold from drywall, you need to determine the extent of the growth -- and that depends on how much moisture is present. If water has been leaking behind the wall, surface blackening indicates a substantial mold colony that is probably also growing on the wall framing and in the insulation. In this case, washing the wall is futile -- you have to remove the affected drywall and throw it away. Nothing will kill the mold growing inside it. On the other hand, washing the wall is an effective way to handle a few brown or black surface spots caused by condensation or high humidity.
Before you undertake any mold remediation project, whether large or small, it's essential to eliminate the moisture the organisms are feeding on. Take care of the plumbing or roof leak that's softening and blackening the drywall, and regulate the room temperature and humidity that's causing moisture to condense on the walls. You may need to run a dehumidifier in the room or increase air circulation by using fans or opening doors and windows; it may even be necessary to re-insulate to reduce the temperature gradient between the outside and inside walls. Shining a light in a dark, moldy corner may help dry it out.
Chlorine bleach kills mold, but before you mix a bleach solution, keep in mind that removing the mold is preferable to trying to kill it -- bleach seldom kills all the spores. You don't need bleach to remove mold; you can do it with soap and water. If you decide to use bleach, mix 1 part with 4 parts clean water, and do not add any other cleaners -- they can react with bleach to release toxic gases. Scrub the wall thoroughly with a sponge to remove all the blackening. If you've taken steps to control moisture, the mold won't grow back. Painting the wall with a paint that contains a mildewcide gives you extra insurance against the mold problem recurring.
Without a coat of paint, drywall paper is porous, and mold on the surface indicates a colony growing in the gypsum core. To control the mold, you have to cut out all the affected sections of the drywall, bag the moldy material and throw it away. The same applies to all the insulation behind the affected areas. After you've removed the insulation, scrub the walls and framing with a bleach solution, and let the bleach remain on the wood for 10 to 15 minutes before rinsing with water. Let the wall dry thoroughly before installing new insulation and drywall. Wear a respirator, goggles and rubber gloves while performing this mold remediation.