You can choose the background color of your rendered movie. The background color only appears when one of your key frames moves outside the bounds of a photo during motion.
You can choose to make the background color transparent, but not all movie formats support transparent backgrounds. Some QuickTime formats which support alpha transparency are the TIFF, None, and Animation codecs when Depth is set to Millions of Colors+. You will need to choose a custom video format that supports alpha to utilize this feature (see Making your movie).