Why tape black garbage bags to outside windows, not between inside windows and blinds?
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.