I also have a small window I will have to blackout as well, any recommendations to do this?
Heavy fabric cut to fit (check fabric store) held up with tension rod. Or use cardboard/foamboard/insualtion panel/etc, cut snug, push into place. Or hang a picture/poster over it! 
We use some black Oracal adhesive vinyl for that. Does not need to be Oracal, can be any cheap black adhesive vinyl.