Black Algae is really a dark blue/green algae form that forms in cracks and areas on your pool surface, and is especially common in plaster finished pools. Black algae likes to grow in shady areas of the pool, and is not very common in vinyl liner pools.
Exception: A vinyl liner pool can develop liner mold which presents as black splotches on the liner, however this is growing below and through the liner, and is not actually algae.
Black Algae contains a heavy slime layer and skeletal growths that make treating it with chlorine or shock resistant. Water will remain pretty clear, though the pool will demand higher chlorine levels than usual.
Treating the pool and removing biofilm will help prevent black algae by removing the biofilm that creates the foundation of the black algae.
To treat Black Algae, have the water chemistry tested. Use a black algae treatment such as Black Algaetrine, and brush the pool before, during and after treatment to allow the chemicals to get into the layers of algae protected by the slimy coating.
Aggressively shock with chlorine, and follow your algaecide directions.
In the case of a Baquacil Pool or pools with alternative sanitizing methods such as Soft Swim or Buguanizes, the Bactericide level should be topped up, and double doses of shock and algaecide should be used.