Sounds like a blood spot or what is sometimes known as a meat spot. A little blood can occasionally be leaked as the egg is released and as the shell forms around the egg the blood is contained with the egg inside the shell. If the other eggs are fine I wouldnt be concerned.
usually the meat spot is just a little dark spot of blood that many people still think is the sign of a fertilised egg. It isnt.
A friend of mine eats battery eggs and still thinks it means the egg is fertile