Slits are a good idea. When you remove a window completely, you let in a lot of light. And if any of those windows are behind you, you can light yourself up inside.
If the inside of the blind is black, you should wear black as well. Something cheap, an old fleece, or a double xx black sweatshirt to go over your regular camo works well. The key is the head, face, and hands. Must be covered.With all of the black, the face, head, and hands really stand out.