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.
Best Answer
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.