I just checked my sample books from 2 different companies and the rubber side is the back, and goes on the inside of the drape or blind...
And if you are saying that the sun is really bad you may want to put a layer of interlining in between fabric and blackout...I highly recommend it and for a few extra $ you will extend the life of your fabric, and add insulating and cooling benefits to your window treatments...I use it almost all the time and when I try to scimp...I regret it later...