No offense, but the gaps in your AE knowledge are as big as the Yangtsekiang is long. The Rotobrush creates transparency, the black is just the comp background, but of course you can enable the transparency checkerboard at any time. That being so, you simply put whatever background you want as another layer beneath the RB'ed layer. This is really basic stuff and rather than relying on poorly done YT videos that may skip these basics, perhaps you can find it in your heart to actually read the help files or at least watch some good tutorials on Lynda.com or even older "Basics" series on CreativeCOW, Video CoPilot and some other places?
Mylenium