I have only done non-medicated blackouts. Usually it's advised that you turn CO2 off during a blackout because the plants can't use the CO2 to grow, and the same with excel.
<Here is a good guide from Tom Barr> that uses no extra chemicals.
Sometimes after a blackout the hair algae goes away completely, sometimes it comes back. I've found that filamentous hair algae just really loves light, and will thrive if the light is too high, so I reduced the light which really helped. After a blackout + a weke or two of lower light, the algae disappears. I've only been able to eradicate hair algae in tanks with lots of floating plants, which reduce the lighting even more.
Excel doesn't seem to do anything to hair algae, but spot treatment for staghorn works well. Once I did a blackout in my nano tank for hair algae, and when I finished the blackout the hair algae was gone, but I had a lot of staghorn. I killed it over 2 weeks by adding daily excel spot treatment with the filter off for 10 minutes after. I think it appeared because during the blackout, the filter got clogged with hair algae and the flow was bad, so then the staghorn came. Before you can treat the algae effectively, you have to fix the causes - which is usually an ammonia spike, too much light or not enough flow.
Have you cleaned your filter recently? Before a blackout you should clean your filter to make sure it's running smoothly with good flow, and then clean it again after the blackout to clear out any algae that was caught in the filter. If you think that the algae is from more waste in the tank, then maybe you need a bigger filter, or need to do to more/bigger water changes and substrate vacuuming to get rid of that stuff.
It would be good to see photos of your tank and where the algae is to see if there are more ways to help. As well as how big your tank is, what the filter and light are, etc.