Speaking from experience, if it's a loosely hanging curtain which will in fact be opened up for the window time to time, you will still have light bleed from the reflection off the BOC or whatever. If it's never going to be opened, seal the window with whatever, such as BOC. You can find BOC at Joann's if it's cheaper there for you, or that you live close enough to one. They'll have it on a roll, and if you have a coupon, it'll probably be very affordable.