Our nursery is very bright, so we have a blackout rollerblind and a roman blind with blackout lining that is a fair bit wider than the window. Together they work pretty well, nothing is absolutely perfect I guess, although grants idea sounds good!
Velcro on window frame you wish to black out, other bit of velcro on double thickness blackout blind material cut and stitched to fit window frame perfectly, so you simply velcro the fabric on and off the windwom frame. Peel on and peel off and the room is a black as black can be.
In the past we've had black out roller blinds and lined curtains but of course you get light leakage around the edges. Is there anyway to really make a room dark ? I can't go as far as external shutters (I'd love to) but short of making boxing for a roller blind, is there a way ? Perhaps an oversized Roman blind ?