The curtains & curtain rod ends look pretty good, but you need an L-bracket bent into a J-bracket for the center to match the end tightness. The curtain sides would be fixed with self-stick Velcro squares. Just those items should get you to almost total black-out.
For total black-out I choose cardboard. Whether it's a single piece or multiple pieces taped & glued together. Even car Sun-Shades or Windshield Shades. The other benefit to cardboard is you can make it decorative by painting it or what I've done is glue-on posters or prints. Turn all of those eyes across the street into a Caribbean Beach, M&M guys portrait or Alicia Vikander.
Use cardboard box sides, not any pieces that have the flaps nor their hinge creases, so the panel stays stiff & flat. You can either Velcro square it over the entire opening or keep the window effect & use the blinds to secure it by just sliding it up & behind the blinds.