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?
No it looks ok. It's not mold growth so idk. Has good veins it it. I'm posting a video now of it but it's hard to see in the box.