Just found this bit on a website Tom.
B. Blood Spots in Eggs
In talmudic times, blood spots occurred in eggs because of two distinctly different reasons. The first was that the egg had been fertilized and a chicken embryo was in formation. The second was that a tissue irregularity in the hen caused a small amount of blood to be deposited in the egg. In America in modern times, since there are no roosters in the egg-laying coop, only the latter occurs, and incidence of that is relatively low (markedly less than 1% of all eggs will have a blood spot).
In addition, United States government regulations require that Grade A and Grade AA table eggs be checked for blood spots in a process commonly referred to as candling (although it is now done with an infrared light) before eggs can be sold to the consumer as grade A or AA. Thus the incidence of blood spotting in grade A or AA table eggs is very, very small in the United States (perhaps as low as one in 1,000). Eggs with blood spots (or other deformities) are marketed as grade B eggs, which are then sold to commercial manufacturing plants to be used as ingredient in manufactured items. The incidence of blood spotting in grade B eggs varies based on a number of factors, and is not regulated by the government. Neither grade A nor grade AA eggs are ever produced in a chicken coop with roosters present, and it is improper to take an egg intended for hatching and sell it as a table egg; it is also very difficult to do, as chicken hatching farms are not licensed to sell eggs commercially to the public.3 Such eggs would never be found in a supermarket as a grade A or AA egg, although if one buys eggs from a roadside stand or a farmers' market, such eggs might be included in the eggs sold