A roller blind, a cellular shade, a wood or faux wood slat blind.

agree with above, but adding plantation shutters.

I think I'd go with a blackout roller blind but also look for those do-it-yourself window film tinting kits that places like Lowe's sells so you have two "weapons" to help block light from coming in.

Any blinds have a possible leakage at edges or with blinds where the cords are so I recommend black out panels.

DD/DH have a media room and it's painted in a dark eggplant color called Raisin( I think). Anyway, a dark color definitely darkens a room. The wood tone color blinds is also a factor in adding darkness.