That has nothing to do with fertilization. Even if it were incubated, by the time it got that big there would be blood vessels all over. You were right to eliminate fertility as a cause.
That looks like a big meat spot. Sometimes hens slough off stuff inside the body cavity and it winds up going down the egg making factory with the yolk. That’s what a meat spot is. That one is unusually big and I don’t know what that sloughed off of. Maybe someone with a medical background could tell us if the liver ever does slough off big pieces. Looks kind of big to be off of a kidney or any other organ I can think of. I don’t think it signifies a sick hen but I’m not a medical expert.
That’s not very appetizing, is it? I get a lot more blood spots than meat spots, but I think it’s a good idea when cracking one of our eggs to add to a recipe to crack it in a separate bowl. I don’t reject many but occasionally I get one bad enough that I don’t want to use it. In theory they are safe to eat, but sometimes the YUK! factor trumps theory.