Hi,
The blackout should knock back the algae quite a bit. Go ahead and do the immediate water change and turn on lights+ CO2 again. The plants need to start production again. Don't forget to dose. I would severely restrict the photoperiod though seeing as that was what got you in trouble in the first place. Even though you are dosing more it takes time for the plants to reconfigure their chemistry to take full advantage of the new dosing scheme. Meanwhile they continue to "bleed" and algae again have an advantage. It's Catch-22 but it has to be done. The frequent water changes help to limit the impact of the "bleeding".
An analogy: If I found you starving in the desert and gave you food and water that would save you that day but it would take a few weeks of feeding before you'd regain your original weight, right? This is the case here. As you continue to physically remove the algae, then also continue to remove ammonia and spores via water changes. Continue to limit the light and to dose. Over the next few weeks you should notice that less and less algae re-grows. This is a signal that the plants are regaining their health. As the plants regain their health they will then start to absorb ammonia instead of ejecting it into the water column.
You've got to think about
why algae flourishes in order to avoid massive outbreaks. Too much light, too much ammonia, poor CO2 and poor feeding. That's it for all intents and purposes. Control these four basic factors and you control the outbreaks. Can you see now why I don't agree with stripping the tank and starting all over again? No matter how many times you restart the tank, if you fail to understand the causes you will continue to make the same mistakes you'll have the same troubles. Many people simply throw in the towel and give up because they never learn to perceive algae as a symptom of plant illness. Instead they perceive it as some foreign invader. Algal spores of every type will
always be in your tank. Like vultures they wait for ill health or poor water conditions to attack. If we focus on keeping our plants healthy we avoid the conditions that signal them to bloom. Avoid the combination of high lights and ammonia at all cost. Avoid starvation by dosing appropriately. Whenever you see algae your default thinking should be - "plants are starving (NPK)" and/or "too much light" and/or "ammonia breach". If you follow this discipline you will always be able to react properly. On the contrary, if you follow the popular thinking that nutrients cause algae, you'll be doomed...
Cheers mate.