Moved from here.
I have these little black spots in my inground pool as well. They don't scrape off at all but have responded to getting smaller when I chalk them with a chlorine tab. not sure what they are either.
Aloha from the mainland and welcome to TFP! Any chance you could post a pic or two of the spots? Could you also update your signature with all of your pool and equipment info as well please? We'll do what we can to help.