[header] How to get rid of blackspot algae in your swimming pool [title] Understand black algae. [step] Like any alga species, black algae (which are really blue-green algae that look black) will grow quickly in warm water on a sunny day. Black algae are chlorine-resistant, because they have a protective outer layer that makes regular sanitization ineffective. 
While algae aren't harmful, their presence can be a sign that other bacteria are present. [substeps] Other alga species that are common in pools are green algae, mustard or yellow algae, and pink algae, which is actually a bacterium.