I agree its the phosphates. I had that black algae stuff a lot. After getting some good advice from this board, I tested for phosphates, and it was off the scale. Did a lot of water changes and finally got the phosphates down to 1 ppm and the algae just kind of went away.
I just check for phosphates once a week and do regular water changes, and I havent had any algae problems (black or green) since then.