My pool has black Algae. It's a 20,000 gallon in-ground SWG pool in central Fl. I didn't notice that the SWG stopped producing chlorine because the salt level was too low. I noticed when I saw a lot of green algae in the pool. I fixed the problem and the green algae was easy to clean. However I'm having a hard time getting rid of the black spots.

PH is 7.4
ALK is 120
CAL is 220
CY is 90
FC is 5
TEMP is 90
SALT is 4000

Now that the Chlorine is back in the pool, do I just keep brushing the black spots with my SS brush for a couple of days/weeks - or do I need to shock the pool to get rid of the spots?
When I brush the spots now (I've been brushing twice a day for 3 days now), I don't really see any immediate change (lightning or reducing in size) but I do think the overall problem is less bad now (less spots).