To repeat what robbo said, my store bought back ground is glossy black on the reverse side. It's not blue on one side though. Just a typical aquatic scene. I bought mine by the foot at Petsmart.
i would strongly advise against the window tint. if you are good with it already, go for it. However, I tried using it on my 220 and a large 6ft piece was impossible to work with. $15 down the toilet. Even smaller pieces like on a 55 would be a great challenge. By the time you got it on and the air bubbles out (if you even could) it would be so creased up it would look worth the time you spent on it. Just spray paint it or by the stuff from Petsmart.