Instead of developing a spiderweb of veins you will see only one around the bottom edge of the egg. Like all the blood pooled to just one vein. It is an indicator that you had a fertile egg at one time but it quit developing on you.
Blood rings occur when the embryo dies. The blood that was moving though the membranes stops when there is no live to continue it. The pooling forms the ring of blood we see when we candle the eggs.