That's what I'm trying to tell you: there is not a point where you'll be able to say, "The black algae is dead".

You can get to, "The black algae is no longer visible, and is definitely not growing", but not to "It's dead."!

You have to hold it at shock levels (adjusted for your CYA level) AND brush it regularly, till it's no longer visible. THEN, you have to maintain good chlorine levels permanently (again, adjusted for CYA) to avoid regrowth.

Do NOT believe the pool ads that say this or that algaecide (usually containing copper) will eradicate it. I have, in the past, deliberately and heavily stained pools with copper, and then watched the black algae simply avoid the spots with the heaviest copper concentration (ie, stains).

Ben