Blood and Meat Spots. I have found some good info in the book Poultry Science and Practice by WINTER and FUNK
Blood Spots and Meats spots may be found in some eggs.They result from the hemorrhage of a small blood vessel located in the small blood vessel in the ovary or oviduct. A blood spot on the Yolk indicates a hemorrhage in the follicle at the time of ovulation.The focille probably did not rupture along the stigma where they are normally no blood vessels. If the spot is in the white of the egg, it indicates a hemorrhage in the wall of the oviduct. Bloody eggs are probably the result of more severe hemorrhage.Th reproductive system of the female is easily ruptured when in production. FRIGHT,HIGH PERCHES and NESTS, and a deficiency of vitamin K may result in an abnormal number of Blood spots.
Hope this might help.