Is black algae in a swimming pool dangerous?

Yes, indirectly. Common pool algae are not directly harmful, but algae convert sunlight into food and release waste products that can feed harmful bacteria and other micro-organisms. Black algae can be quite difficult to eliminate, particularly in plaster pools. Any item that has been exposed to the algae should be treated along with the pool water. All pool toys, nets, hoses, brushes, etc. need to be thrown into the pool and treated. The sides of the pool will often need to be scraped or brushed. Probably the best algaecide is copper (II) sulfate. After the treatment is complete, you may then need to use Pool Magnet to remove the copper from the water.