I have a gray plaster pool and noted black spots in two areas of the pool. I have scrubbed bb daily and treated spots with "spot kill". In the morning there are occasional spots that reappear but as soon as the pump runs they disappear. I plan to continue bb daily for a week and then q week. I shock the pool weekly with 10% bleach ( I had high CYA levels and after draining 2/3 water they came down to 40). Water analysis pH 7.6, FC 3.5, TA 120, CYA 40 and has remained pretty stable for 2 weeks. I was told at store that I will never get rid of it and should empty, acid wash and start from scratch.
A post somewhere else suggested treating it with approx 8 gallons of 10% bleach followed by two gallons of Ammonia Hydroxyde, but I am afraid of just adding stuff.
It is a lot of work daily and I am afraid it will continue making my pool ownership more work than fun.