I hate to be a downer, but the lower left of that egg does NOT look normal to me, especially if it wasn't like that when laid. When people talk about a "boob" egg, it's when part of the shell doesn't fully form and is almost clear; again, it does not happen after the eggs are incubating. Can you tell whether the dark "stuff" is actually a color change of the egg, or is it possibly mold growth that you can wipe off?