Black algae is a sign you need to shock harder.

Low chlorine and poor circulation.

Black algae forms in dead zones — corners, behind ladders, steps, or where water barely moves. It also loves low chlorine levels. The algae itself has a waxy outer layer that shrugs off normal chlorine like a raincoat.

You can’t kill it with a regular shock. You need to brush the spots aggressively to break that coating, then hit them with a high-dose chlorine shock (or a yellow/mustard algaecide if you want the easy route). Keep the filter running 24/7 and brush every day for a few days.

If it keeps coming back, check your phosphates. High phosphates feed algae even when chlorine looks fine. A phosphate remover can starve it out.

You can win, but it takes persistence. The algae is patient. You have to be more patient.