Get some hanger bolts, lag screw/machine screw combo. Install the hanger bolts lag screw into the inside of the window frame every 10-12 inches. Get 1/4 inch masonite hardboard and some decent weather stripping. Drill holes for the machine screw to come through the hard board. When you want it dark slap up the hardboard on the inside of the garage with the weather strip between the window framing and the hardboard, apply wing nuts tighten until dark.
That way you can take it on and off quickly.
My wife, must have been close to 20 years back, wanted me to put curtains or some crazy thing to hide a blacked out window. She required black fabric covering on Oak plywood? She didn't want the neighbors to see the oak.
I have found a low dose of Valium or other mild tranquilizers come in handy during these times.
Leave up your black out so your wife doesn't require a paint job or curtains to hide the masonite.