Some ideas, huh?

1. Maintain chlorine levels per the cl-cya chart: http://pool9.net/cl-cya/
2. FOR CONCRETE POOLS ONLY: brush algae spots with an SS bristle brush, regularly.
3. Maintain a low phosphate level. (Only works if you are ALSO doing #1 above!)

There are other products that will kill black algae, like monochloramine or strong copper algaecides . . . but they have severe side effects.