This is my 18 week? Jersey giant. The eggs I've been getting are fertile ( bullseye is very prominent) and I think he is my rooster. I don't own any roosters whatsoever. But I think that he is. Hes got the cock stance and a hen squatted for him! He also bosses them around.
How long did that egg sit in the sun? A few hours? Or a few days? It takes at least 72 hours of being kept at a near constant 100* for veining to be visible.
Most likely it was just a blood or meat spot in the egg.