I am new to this too. I am setting up my 125 gallon tank right now, and was thinking of what to do with the back as well.
Following some posts about the subject, I tried a black plastic back (cut down the sides to fit), last night. My first try looked pretty dumb. I used Target brand Lawn and Leaf bags, and you could totally see through the plastic. Smokey grey with plumbing showing through, -UGLY.
Then, I tried black contractor refuse bags from the hardware store. I remembered that these were amazingly thick, we put plaster and lathe in them when we remodeled our house. Also, they come VERY big, so there was no trouble with fitting it to the tank.
This worked really well! I used contractor grade black electrical tape to seal the edges. You can't see through it at all. Very black.
I was worried about painting because I have a parrot in the same room and they are very sensitive to chemical fumes. You know, the canary in the coal mine thing. (And my husband dreams about putting the tank in a wall someday, so we would need both sides clear).
Just my experience. Good Luck.
-Christine