Blackout is total darkness or it won't work.
helping a friend i took a picture of the tank wrapped
wrapped the sides first and pulled the plastic under as possible taping it to the bottom. then wrapped the top and sides, don't worry you can't seal it air tight.
what we did on granny's tank was clean the hob filter, trim the worst of the bba off the plants and did a water change. the hob and an air stone ran during the blackout. wrapped the tank on Monday night and removed the plastic Friday morning. no peaking! fish were hungry but all did fine making it through the long night.
granny had neglected the tank for about 6 weeks, dirty filter, badly overgrown, bga on the gravel, bba on the plants, thread or hair algae too. tank even smelled bad. she didn't want to use chemicals and had nowhere to put the fish to break the tank down.
a month later it looked really clear and the plants were filling back in. her son does water changes for her now every 2 weeks and the tank is back to low tech easy and stable.
start a blackout with clean water, clean filter and don't cheat any of the steps and it will kill off a bunch of algae. vacuum the tank afterwards then dose the ferts and address the original cause of the imbalance.
from the one handed lol
hth