I'm not sure of the exact and complete answer; but blood spots have to do with the egg having been fertilized, I believe. Had the egg been incubated a chick would have been produced. Eggs we buy at the store almost never have these "blood spots" because the hens aren't allowed even close to a rooster, so are never fertilized.
Offered by Ron.
From an egg FAQ.
Q: What causes blood spots?
A: Small spots of blood (sometimes called "meat" spots) are occasionally found in an egg yolk. These do not indicate a fertile egg; they are caused by the rupture of a blood vessel on the yolk surface during formation of the egg. Most eggs with blood spots are removed during the grading process but a few may escape detection. As an egg ages, water moves from the albumen into the yolk, diluting the blood spot. Thus, a visible blood spot actually indicates a fresh egg. Such eggs are suitable for consumption. The spot can be removed with the tip of a knife, if you wish.
Offered by George.