I've recently been working through the various reflection parameters and how the various colours interact. I'll be putting this up on my site in the next day or so. In the meantime, yes, Background should be black. It appears to add to the surface colour of the reflective object. If your reflecting object is red, for example, and you set Background to white, the reflecting object is coloured pink. Black on the other hand does not affect the surface colour of the reflector.

Steve