It’s perhaps the most aggravating strain of algae, it has been compared to herpes; “once you’ve got it, it’s there for life.” One can only manage the problem with treatments. This is not entirely accurate, but the difficulty in eradication is due to the strong roots and protective layers over top of the black algae plant.
Black algae will appear as dark black or blue/green spots, usually the size of a pencil eraser tip. It lurks in cracks and crevices in plaster, and is one of the hardest to remove. Their roots extend into the plaster or tile grout, and unless the roots are destroyed completely, a new head will grow back in the same place. The heads also contain protective layers to keep cell destroying chemicals from entering the organism. Like yellow algae, black strains can bloom even in the presence of normal sanitizing levels and proper filtration.
Black algae is more typically found in concrete or plaster finished pools; it is known for a heavy slime layer and “skeletal growths” that make it impervious to normal chlorine levels (1.0 – 3.0 ppm). Black algae can grow “condominium style” providing layers of algae one on top of the other. Slight cracks in plaster are perfect breeding grounds for black algae especially when if the pool surface is near or past its life span.
You actually cannot get rid of black algae unless you get rid of the spores that propagate it.