First photo will give you an idea of the relative size of the black spots, second photo is macro and is better to see the actual spots. This is located on the 1st and 2nd steps on my fiberglass pool. The variation in color is from paint that was applied 11 years ago and has seen better days.
I read that cobalt is raised and can be scraped off, but I cannot tell if these are raised or not. When the pool was painted the contractor sprinkled some sand granules on the steps to provide some traction so the whole area is finely textured. I have not been able to scrape any off, but have not tried too hard.
I have several other spots in the pool, all on the walls, where I am pretty sure I have some cobalt staining, but these areas are bigger- just a little smaller than a dime, and there is only one or two in the area, not so many tiny dots.
I intend to raise my CH this weekend, shooting for around 300 based on my reading- unless someone has other advice. Does the calcium actually reverse (remove) cobalt staining or just prevent more from occurring?