What does it mean if I have black algae in my pool?

Your pool likely suffers from problematic filtration, as black algae usually grows in spots where circulation is poor. There may also be an issue with the chemicals and pH of your pool. To correct the problem, you will need to give your pool a little extra attention. Brush the surfaces thoroughly and test the pH of your pool, correcting this as instructed by your kit. You may also need to use an algaecide. To ensure that it does not return, maintain your pump, filter sand and pool circulation.