Black algae is the most difficult type of algae to deal with. Unlike other forms of algae, black algae releases spores that will remain dormant in a pool. As soon as sanitizer levels begin to drop, black algae blooms form. The black algae blooms themselves are also notorious for being tough to kill.
These spores are incredibly resistant to oxidation, and have been known to even survive acid washing. Luckily, spores released are invisible to the naked eye. However, this means that once your pool has black algae, the pool will always have these spores waiting. Rather than simple eradication, this becomes a game of prevention to keep away visible growth.
While black algae can grow in any type of pool, it is more common in plaster/gunite where it grows into the porous surface.