One thing to keep in mind is how obvious do you want your blackouts to be? There is a house on the highway a few miles from me and at random times they put up Styrofoam in their windows and at night small amounts of light get out(the Styrofoam is up for a few days then down for a week then up for a week, then down a day and so on) The way they do it makes the house look very suspicious. If they left it up all winter it would make sense but the way they do it makes it appear as if they put the Styrofoam up when they do something in the house they don't want to be seen doing. It may be totally on the up and up but because of the area and the condition of the home the first thing I think of is they are cooking meth, and others make the same joke when driving past it. In normal conditions their blackout method makes the house stand out even more.
My plan is in several stages.
Close the curtains then hang heavy blankest over them. They can easily be removed and from outside it doesn't look odd.
If thing get worse, close the curtains then put up sheet metal with the white side facing out on the inside of the curtains. This adds a bit more security, still doesn't look odd from outside and if done right blocks all light. If needed a small peep hole can be drilled in the sheet metal to look out. When not in use the hole gets covered.
If things get worse yet the sheet metal would go on the outside of the windows to protect the windows. This will make the place look obviously boarded up which may be a good thing or bad thing depending on the conditions
I haven't done it and don't intend to do it on this house but light tubes
https://www.amazon.com/Natural-Ligh...ocphy=9019361&hvtargid=pla-568081365822&psc=1 Look like they could be a good solution to letting light into a boarded up home without giving any sign someone is in the home.
I have also thought depending on the situation I may put up sheet metal on the windows but leave the top few inches open. Light could come in during the day but at night I could block the small gap at the top to make sure no light comes out.
If you do block your windows it is a good practice to go outside at night with the interior lights on and see where light is leaking out so you can block it. A small sliver of light may leaking out may draw more attention in the wrong circumstances than an totally uncovered window and light would.
The darker your area the better you light blocking has to be. If you live in town and their are working streetlights you can let out quite a bit of light and no one will see it. If you are in an area where there is no other light the glow of a candle through a nail hole can act a a beacon for a long ways.