What is the only thing that works for black algae in a pool?

Brushing is the only thing that works. You need a stainless steel or wire brush to break the black algae’s protective outer layer, then hit the water with a heavy chlorine shock and a copper-based algaecide. Repeat the scrub-and-shock cycle three or four times over a week, vacuuming to waste after each session. Your elbow grease is the active ingredient — chemicals alone won’t win.