We have blackout cellular blinds. It is amazing how much light comes in thru the small gap at each end. I wake up at dawn or when the yard light goes on. I saw these, thin, black non-reflective inside – Exactly what I was looking for. I’m a DYI type, some say too perfectionist, so I took off the magnetic strip and used very small flathead screws. If the gap blockers fit in the gap between the shade and the blind with the magnetic strip, even less light would come through. If I were to do it again, I’d go with the magnetic strips first. Either way, they must be removable incase you need to remove the blind. They work very well at darkening the room. But with cellular blinds and the need for some gap at the ends, it is not possible to be completely dark. This goes for the door too, when the room is very dark the small crack at the door bottom lets in an amazing amount of light. All-In-All it is much darker at dawn and I am very pleased. I am quite surprised at how well they blend into the window, you don’t even notice them.