Thanks for the quick replies!
davidfor, I totally believe that some covers have top-and-bottom letterboxes, all my covers just happen to have a narrower aspect ratio than my device's screen. "Pillarbox" is a term only pedants like myself even know, "letterbox" is absolutely the right term to use
NiLuJe, thanks for the dive into the code. Does a "hard-coded" black absolutely preclude the possibility of a different color, or does it just mean that the modification would be less trivial than it first seemed?