"Black mold" refers to a strain called stachybotrys, a variety of mold which produces asexually.  The mold is typically associated with poor air quality, and can adverse health effects.  Depending on the length of exposure, symptoms can vary from minor irritation, to difficulty breathing and bleeding from the lungs or nose.  A substantial body of medical and legal work has been established dealing with the effects of stachybotrys mold species.

Black mold is found mostly in damp or wet areas. I used to live in western Washington state and black mold was common in old buildings. A friend of mine moved into an apartment with her boyfriend and they noticed black streaks on the wall. The landlord said it was because they had a gas stove (which makes no sense). Anyway, they were sick the entire time they lived in the apartment and once when cleaning the couch found a thick layer coating the entire interior of the couch as well as back of the couch. They moved all their furniture and black mold coated all their possessions. Check out symptoms and pictures here:

http://www.blackmoldsymptoms.net/