Black algae is the most stubborn algae to get rid of in swimming pools. Using algaecides sounds good, but in my experience they don’t work that well and add metals to the pool water creating the possibility of staining plaster. The effective ingredient is usually copper which used over a prolonged period of time will “copper plate” the plaster on a gunite pool.
The most effective method to get rid of black algae is to take a 3″ chlorine tab, get in the pool and using a mask or goggles and something to hold you down (ex:20 lb. rubber coated dumbbell), rub the chlorine tab directly over the spot. This will lift the black algae off and leave a little chlorine in the pores of the plaster. Wait a day, then using a metal brush (preferably a 6″ brush as it’s easier to apply more pressure), brush over the spots that have been removed. Vacuum all algae out of the pool and backwash the filter. Get it out of the the system!
It’s important to start early on before it takes over the entire pool. If the black algae has already taken over the pool, the most effective way is to drain the pool and spray bleach directly on top, let it soak for a day, then use a pressure washer to remove it. Remove all traces of algae & bleach and refill the pool.