I don't know how a black background can decrease the amount of algae, since the major light source is inside the tank, not outside...
Anyway, for the black acrylic, check out sign marker in your yellow page. They have acrylic and other plastic in every color anyone can imagine. Also check with plastic places like USPlastics.com and others.
In addition, if you still have hard time getting them, post a message in WTB forum. I am sure some of us have scraps laying around in the garage somewhere.