Can a newbie chime in?
I am an eggs-pert on eggs--I have my own chickens, and I used to work as a poultry-egg inspector for USDA.
The blood spots are NOT a sign that the egg is fertilized. They occur when the hen has an minor injury in her reproductive tract, and the blood is incorporated into the egg white. Sometimes there will also be little brown specks, called meat spots, which are actual reproductive tissue.
They are more common in brown eggs you purchase only because brown eggs are harder to candle. Candling is looking at eggs with a strong light source that illuminates the interior. Eggs with spots are taken out, and sold as baking eggs.
Fertilized eggs have a teeny little white circle on the yolk, which becomes visible after the egg has been incubated or kept warm (99 degrees) for 24 hours. After 48 hours, a system of blood vessels develop, and the yolk looks like a bloodshot eyeball.
Some home producers candle eggs, especially if they are selling at a farmers market. I don't candle mine, but I do collect them every day, so there is no chance of embryos developing.
"Home made" eggs taste sooooo much better than grocery store eggs. My girls roam all day, eating bugs and seeds, and their yolks are big and bright orange. And of course they are super fresh--grocery store eggs may be kept in cold storage for a month before they are packaged.
Probably more egg info than you needed.