It seems as though you have somewhat similar situation to mine. I have a hard time removing the black spots as well. It was much worse a month ago, and just like you, I used to think my gunite surface is wearing off, and just let the algae grow slowly on the walls, finally when I realized, I got a stainless steel wirebrush and started scrubbing off daily. I got it to a point where it doesnt go off anymore, though there are still some spots. You could read my post here, http://www.poolforum.com/pf2/showthr...otty-and-dirty
I still havent found any real easy solution to knock the black algae off the walls, and scrubbing doesnt remove all of them, so I am still working on them, you can see pictures there as well. Keeping high FC doesnt seem to make it easier to remove them since they seem to have a much stronger root on the walls and chlorine doesnt seem to affect the inner side of the algae colonies. Probably you could learn something from my experience and we could share some info which will help me as well. I am at Phoenix.