I have come up with a way to blackout windows that is 100% effective and is quite cheap and easy to do. I thought I'd like to share my instructions. If folks like this idea, perhaps it's worth being stickied or linked from a larger idea thread.
The basic idea is to put up roller shades in the window, use magnetic strips to seal the edges and cover it all up with mini-blinds (or curtains if you prefer). The beauty of this technique is that it's easy to apply and remove the blackout and it's practically invisible in both cases.
I did this in my office because I like playing computer games in the dark, but you can use this on any window. I had considered buying a system to do this but I found them both expensive and intrusive, requiring large boxes at the top of the window and unpleasant rails down the sides.
Here's the window I converted:1) Obtain roller shades for your window.
Begin by measuring the top inside width of the window frame and subtract 1/4" from your measurement. Take that measurement to Home Depot or Lowes and have them cut two roller shades (the cheapest brand is fine) of that width and the height of the window (you'll want at least two inches extra in height). Now, you are going to be modifying the roller shades they give you, so don't follow their instructions; they will ask you for the inside width of the window and cut based on that, but if you do that, the vinyl on the roller shade will leave a gap of 1/2" to 1" on either side of the window which is too large. Tell them you want it cut as an outside mount or just tell them to make sure the vinyl (not the dowel) is cut to your measurement (the width-1/4" value).
You want two shades because in my experience, one isn't enough to completely block out the light. With the thickest of the cheap stuff, you'll still get an orange glow through the material. If you pick two slightly thinner shades (I think they are 1 or 2mm thinner; I can't remember but I think they were something like 5mm thick), that will be good enough.
Also ask them not to put the cap back on the end they cut off. You'll be doing some more cutting.2) Modify the roller shades.
The roller shades you have now will not fit in the window. The vinyl should just fit, but the dowel will not. To deal with this, rip the vinyl off both dowels. Make sure you put a marking on the dowels so you can remember what direction the vinyl was rolled up. If you get it backwards, then the roller shade will just unroll when you install it. Next cut the end off one of them (one end should be uncapped from when the store cut it) so it'll fit perfectly in the window (don't forget to include both caps in your measurement).
Overlap the two vinyl sheets. You're going to roll them both up on to one dowel. It's important you don't glue the two sheets together. Because one rolls up outside of the other, the sheets need to be able to slide against each other as they are being rolled or unrolled. Make sure that when you unroll them up, the ends are even. You'll want to cut the end off (that has the plastic strip in it) of the sheet that rolls up on the outside; this is so you don't have two plastic strips to mess with.
Apply some glue to the top edges of the sheets, center them on the dowel and roll them up.
Also pay attention to the tension in the springs inside the dowel. The tension is set when you buy it so you can install it and pull the shade down. If you install the shade, pull it down, take it off the window without rolling it back up and then roll up it. Once you install it the second time, the spring will be too tight and you won't be able to pull the shade down, or if you do, it'll probably flip back up. You can also make the reverse happen where you loosen the spring so that there's not enough tension for the shade to roll back up when you tug on the bottom.3) Glue a thin metal sheet to the top edge of the window.
It doesn't have to be metal, but thin aluminum is probably the best thin thing you can find. This will be used to seal the top of the window. You'll want the sheet to extend across the width of the window at the top and be about 3 1/2" high. I painted the outside of the sheet white and the inside black. The sheet won't be visible from the inside, but it will from the outside. Use glue or caulking to seal the edges of the sheet.
You can see the metal sheet below. It's not pretty, but it's mostly hidden.4) Attach magnetic strips to the top and sides of the window.
The top strip will run along the bottom of the metal sheet you just attached. You should use high energy strips as the stuff you'll likely find in a local store won't be strong enough.
I purchased a 1/2" roll from this company here: http://www.magnetics.com/product.asp?ProductID=39
. I recommend the strongest they have. They have self-adhesive strips which is nice.
You can see the top strip in the above image (if you look carefully). Here's another of the edge:5) Install your roller shade and mini-blinds.
Make sure that the holders for the roller shade are far enough from the window that the roller shade will fit when it is fully rolled up. Also make sure that the mini-blinds are installed at the outer edge of the window so the roller shade will fit behind it when fully rolled up.
Here's the window with the roller shade pulled down:6) Black out the window.
To put all this to work, you simply pull down the roller shade and attach a second set of magnetic strips (just leave the adhesive backing on the strip). that will clamp the shade down and seal it to the window. The top strip will seal it to the strip on the metal sheet. You'll want to pull the shade down several inches farther than the window so the bottom folds out onto the window sill. I made an attempt to attach these second strips to thin metal strips to make them easier to put on but I found that leaving them unattached is much easier and more effective.
If you've made the metal sheet low enough, you should be able to do this even with the mini-blinds installed and pulled all the way up:
I have found that the roller shade can wrinkle up a bit and simply setting something like a wooden or metal bar on the bottom, holds it down and seals the bottom:
Once you lower and close the mini-blinds, everything behind them disappears as you can see from the first picture at the top.7) Sealing the door.
Chance are, you also have a door that lets light into your room. Here's how you deal with that.
First, I bought some simple strips of adhesive foam weather stripping and installed on the inside of the door frame:
In my case, I have french doors which also allow light in from between the two doors. Two deal with that, I bought a thin aluminum strip, painted it, and attached it to one of the doors so it overlaps the gap.
I used left over wall paint to paint the strip so it would match the doors but the paint doesn't stick to aluminum very well. You might be better off applying a coat of spray paint to the strip first and paint over that.
It's easy to drill holes through the aluminum. I pre-drilled holes in the door too so I wouldn't splitter them when installing the screws.
On the underside of the aluminum strip, I glued a strip of felt that provides a soft padding to hit the other door with. You can glue the felt on before attaching the aluminum strip to the door with part of the felt beneath the door and the aluminum strip.
As for the bottom of the door, well I petered out here and decided not to do anything about that. The area outside these doors is relatively dark and very little light actually comes in under the door. If you completely block out that gap, you may have trouble with ventilation in the room and it may cause whispering or vibrating noise as air from your A/C escapes. If anyone has any good ideas for this, feel free to add them.8) Examine your results.
Here's what the window looks like with the roller shade pulled down, but without the edges sealed (and this is with full sunlight shining right on the window):
And here it is fully sealed (you'll have to take my word for it ;-). That little red dot in the bottom corner is the light on my mouse.
And with my monitor for a bit more perspective.
All in all, it should cost around $150-$200 ($100 is the minimum order if you buy magnetic strips from that company I mentioned...but then you could do multiple windows). Once you're finished, it takes about 30 seconds to black out the window and less to undo it and it's virtually undetectable.