When I did a blackout I did not wrap my tank up so no light would get in. But there was a die back of the HA and my hermits eat very well in that time. Once they reach a decent size they kept the HA down to 0 in the tank there is always going to be roots and HA will grow if algae eating inverts and snails are removed. But that is why they are in our tanks in the first place to clean up what we do not want in the tank. So this is what I did reduced HA so there was next to none and let the cleanup crew do there job. When Hermits die off replace what dies off or was eaten by larger hermit. This has kept my tank nice and clean I also have a fuge in my sump which helps remove the phosphates.
Bill