I wonder how to block the light from a window that spans up to the ceiling. I want the room to be as dark as possible.
Here is the setting:
An additional constraint is that I cannot remove the blind that is adjacent to the window. (I can remove the other one.)
As you can see, there are currently 2 curtains that do a poor job at blocking the light, as they let light comes through the top, the left and right.
If you want a complete solution for the whole window, or want to block the light just at the top, you can tape aluminum foil on the whole window or just at the top of the glass. I did that once in a room that got so much sun that it was always too hot. It took the temperature down about 10 degrees.
There are light blocking cellular shades the can be mounded toward the room side of the venetian blinds. You might need to construct baffles at the top and on the sides as well that the shades rise behind.
You also could get a light blocking roller shade, place it room side of the blinds, and have the shade run down channels on either side. You still would probably need a baffle for light leakage at the top.
Images and links are for illustration only, not an endorsement of goods or sources.
Are they blackout curtains that you are using? Also getting a curtain rod that wraps would help like the little white cheap ones from walmart. Those also stay closer in so they block more light from the top.
Issues I see:
cheap blinds - get thicker blinds
blind slats too narrow - get wider slats on better blinds
the blackout curtain is light grey - I have gone through this. The light grey kind of works but not like the black. The fix is to go to black blackout curtain or you can go with what @Johnny mentioned in comments and sew some velvet or thick black material to the back of the curtains.
there is a gap at the top - you need a curtain valance. Preferably going to ceiling and made out of blackout material double up.
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.
