The icon that gets displayed is based on the model identifier (see System Profiler; my Blackbook (Late 2007) has an identifier of MacBook3,1).
Both white and black Macbooks share the same model identifier, because they use the same motherboard; this is in contrast to the difference between the 15" and 17" MacBook Pros (for instance, the first MacBook Pros were MacBookPro1,1 (15") and MacBookPro1,2 (17").)
There are icons for both the white and the black Macbooks in /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources; it's just that there's no way (that I know of) for the computer to figure out whether it is white or black.