On other things, the background is dictated by the mangaka so if its filled in black by the assistants its either because the mangaka or probably the editor said so. Or it might be some sort of Reality Marble!!!
whats with obito's what I want is to be whole again? is that because half his body is probably white zetsu or is it because he had to sacrifice something in order to go on living? or does he simply want his eye back from kakashi even though he's made no indication that he's looking for it.