How do I permanently get rid of black spot algae in my pool?

You can’t scrub black spot algae away — you have to fix the underlying nutrient imbalance. Test phosphates and use a phosphate remover if above 100 ppb, then shock with liquid chlorine to 15–20 ppm for 24 hours. Keep pH between 7.2 and 7.4, run the pump longer in hot weather, and brush concrete weekly to disrupt roots. If it returns, check for circulation dead spots.