This thread makes no sence. It isnt any harder to clean the back then the other parts of the tank. Takes 2 minutes during water changes. Just my two sence the answer being looks like crap just the same as on any color back ground. Do a google search of tank algea im sure you will find plenty of pics. Sence most tank with plain color back grounds are usually either black, or blue.
When you have tanks back to back if you leave the back's clean than the fish tend to try to "interact" with the fish in the opposite tank. This leaves fish in the tanks on both side more interested in the back of the tank (opposite of the viewing front surface). When there is a thin film of Algea on the back walls of the tanks the fish do not focus on the tank that borders it's backside. This gives you many many times better view of the fish in both tanks, rather than look at your fish all the way in the back of your tanks trying to attack the tank adjacent to its backside.
I despise backgrounds, but would 100% require them on the back sides of tanks to keep the fishes attention on anything oter than the opposite tank!
I don't think that Algea looks like "crap" though. I think it looks absolutely awsome and find it greatly desireable, just not on the surfaces that you are viewing the tank through. I doub't that there is even the space for my skull "between" the backs of the tanks to attempt to "view through the algea on the back surface"