Most fish shops sell aquarium background by the linear foot. A common one is black on one side, blue on the other. Just make sure you get one tall enough as they come in different heights. Black foamboard usually costs more, but works too. Its sold at Staples, Walmart, etc for presentations/ projects. Even cutting a black trashbag/ contractor bag could work but may have visible folds in it.
My #1 choice (besides paint which is out) would be the store bought aquarium backer. Just cut to fit and tape the corners with little pieces of scotch tape.