I bought piles of military surplus camo netting at a gun show years ago. I hung the camo on the inside of the windows. You can still see out of it. I put up small nails to hang the camo out of the way when I want a small opening for my rifle. I also agree with the idea of painting the inside of the blind black. That produces what I like to call the puppet box or shadow box effect. It effectively cuts off any back light that will produce a profile of the person in the blind.
I bought all my windows for my covered blinds when my wife was working at Menards. I waited until the windows were on sale and then got her employee discount on top of that. I jokingly said that for a while I had better windows in my shooting houses than the house I lived in. The sad part was that it was true!