Hi Altantuya, thanks for asking this question. It's a fairly common one! Here's the answer:

Blood spots are occasionally found on an egg yolk. They are caused by the rupture of a blood vessel in the hen when the egg is forming, or by a similar accident in the wall of the oviduct. During the grading process, candling methods reveal most eggs with blood spots and those eggs are removed but, even with electronic spotters, it is impossible to catch all of them.  As an egg ages, the yolk takes up water from the egg white to dilute the blood spot so, a blood spot actually indicates that the egg is fresh! Both chemically and nutritionally, these eggs are safe to eat. The spot can be removed with the tip of a knife before you cook the egg, if you wish.