56 Pack Sound Proof Foam Panels |1"×12"×12" Quick-Recovery Soundproof Wall Panels | High Density Sound Dampening Panels for Home Studio |With Adhesive Stickers 280PCS (Black)

Maybe adding acoustic panels, something like acoustic sound proofing panels A little unconventional, but instead of putting them on the walls, maybe attaching them to the inside of your blackout curtains? You wouldn't see them, but the panels between the curtain/windows might help a tiny bit more in reducing sound.