They do make black foamcore. It's what I've been trying since I got some someone was going to throw out. Unfortunately, the pattern I made of the window by pressing butcher paper into it then tracing with a pencil was less that perfect. I think when I trim the edges that are too big, the ones that are too small are going to cause it all to fall out.
Foamcore is basically polystyrene sandwiched between paper. Paper can go moldy under the right conditions, but so can most things. There's a fungus for damned near everything.
As far as color, the black foamcore was perfectly stealth viewed from outside. I have a slight tint on my windows...not enough to not use window covers. The black foamcore was matte style so it didn't reflect light. If I could just get it cut to the right shape, it would be perfect for cover. Not sure how great it would be at any insulation, though.
I used wide heavy-duty foil to imbed the shape of my windows before cutting the shape and tracing onto reflectix. It worked quite well. If in doubt, cut it all just a little larger than your marks, then trim.