Black algae are single-celled organisms that grow in large colonies. They contain chlorophyll like other algae, but they also contain compounds that mask the green color. It forms in cracks and crevices on pool surfaces (especially plaster finishes) and can grow somewhat protected from the surrounding environment due to the formation of a protective layer on the outside of the cell. This makes black algae more difficult to treat and it is somewhat resistant to normal chlorine levels.
IMPORTANT NOTE: Brushing is extremely important when treating black algae because that protective layer has to be broken in order for the sanitizer or algicide to come in contact with the cells.