Black mold grows very quickly, often leaving stains if it is not removed properly or quickly enough. Black mold is known for having especially strong roots and multiple protective layers, making it one of the trickiest molds to kill in a pool. Black mold or algae looks like dark black or blueish-green spots, and its roots will often grow out into the tiling or walls near the pool. If not completely eradicated, black mold will also grow back again.
Brush any black mold formations thoroughly with a stiff, wire brush.
Turn off your pool pump.
Use a large chlorine tablet and break it into a couple decently-sized chlorine tablets. Rub the chlorine tablet onto the areas of black mold. Do not use chlorine tablets if your pool is lined with vinyl.
Pour a generous amount of a good all-purpose algaecide onto the areas of black mold. Let this sit overnight, with your pool pump still shut off as well.
Scrub off the black mold areas with your wire brush. Use a wet-vac to suck up the nasty black mold sludge into a safe disposal area. Start your pool pump again.