What is a good material to block all light from a window with?
After failing to find a solution that would (1) look alright, (2) not devalue the house, and (3) be permanent, I looped back and looked to blackout curtains more. The solution I ended up with involved:

Getting blackout roller shades cut to size, installing them myself.
Buying floor-length blackout curtains and sewing magnets into the seams where they meet, as well as attaching velcro to the sides that keep it snug against the sides of the window.
Creating a cornice box to block light coming in from the top.

This ends up working very well, with some minor light spillage on the bottom that's very hard to notice while lying in bed. Best of all, it was inexpensive to do and looks great when opened or closed.