I went store to buy double layer of heavy duty black garbage bags to black out a bright bedroom at night, and spoke with salesperson and his supervisor. They both recommended go outside to balcony and tape these bags on outside window. Don't stay inside and don't tape bags between blinds and inside window!
My English is bad, and I can't remember their explanation. Can anyone think why? Because weather? Condensation? My condo's in Toronto, Canada. Toronto snows and gets cold in winter.
Because black surfaces absorb sunlight and convert it to heat. You don't want to accumulate that heat inside the house, usually. Of course, this assumes that you don't install and remove the covering daily.
You could also use aluminum foil and avoid most of that issue. However, check where the light shining on your window will reflect to before using aluminum foil (you might annoy your neighbors or fry your plants).
Caveat (thanks, Max, and others who confirmed): With double or triple glazed windows, it can be destructive to mount bags or reflective foil from the inside. It can cause uneven expansion of one glass panel which can result in fractures or breakage of the window.
It does work better to have them outside. The filtering in the bags + window after is better than window + bags. I have actually tested this.
Outside though like Ed mentions, really really hard to secure these.
Inside they can kind of smell and it looks really bad - sunlight heat sits between bag and window.
Those are your main pros/cons.
If this is long term I would think about tinting solutions or shutters that close.
If you are doing this for blocking light I would possibly do something different. I used aluminum foil. I applied it with a water sprayer on the glass and wiped it down. It only had to be taped at the edges and the window could still open. If you want to open the window or door, often 1/2 the glass being sealed on the inside and the other half on the outside with whatever you use is the best option. Being careful I have foiled the insides of my windows for 100% light blockage but sliding the window scratches the foil and it will slowly let light in over time.
As others have answered, the main consideration is indeed where you are trapping the heat. Even in watery, winter sunlight, you'll be amazed how much heat ends up being absorbed into the black plastic.
Depending on the glass though, it may not make all that much difference heat-wise - especially once the plastic is saturated with heat, heat will be radiated through the glass. Although, with the plastic on the outside, you should expect less of it as it will be cooled by any breeze, and some of the heat will be radiated away from the glass too.
It's really a matter of compromise - it's likely to be trickier to affix the plastic outside, it's also going to get wet, wind is going to bother it, and you may even have insect issues (here in Australia, we definitely would!).
If this was me, I'd be installing blackout blinds - you can open them easily to let light in when you want and there won't be maintenance issues. However, there is of course a cost issue (they sure aren't cheap) and it's not an option for everyone (renting, etc). So if I couldn't install blackout blinds, I also wouldn't be putting foil on the outside - as effective as it is, it'll be bright and ugly. What I would consider, is using dull foil (like the insulation foil used behind plasterboard) as a layer on the inside of the glass to reflect a decent amount of energy, and then use the black plastic on the inside behind that to block remaining light if desired.
We covered inside window with black garbage bags to temporarily darken room. It was really cold outside - 35 C. When we removed the plastic three days later, the window had cracked. Not sure why but likely something to do with the temperature difference?
