black cardboard & double sided velcro sticky dots ( you can buy them in a newsagent) & you can put them up or take them off as you like.
I have two big ones for the windows in ds#1 room as his curtains are ok, but not block out, I will do the same for #2 if/when it becomes an issue.
Any residual sticky stuff from the dots on the window/windo edge, will wash off with metholated spirit