Regardless of what your black spots are, with CYA => 100+ ppm, you *must* (a) read the "Best Guess Chlorine" page and (b) get a K2006 test kit. You can not keep a pool algae free at the chlorine levels possible with a conventional test kit. Links for the Best Guess page, and for the K2006 on Amazon are in my signature.

But, to firmly determine what the spots are, swim down, and scrap the spot with your fingernail. If the spot IS black algae, you'll end up with a bit under your fingernail. Swim up, and smear the algae on a bit of white cardboard or the unlined side of an index card. Black algae will look almost black, but will smear dark green.