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.