By Rabbi Julian Sinclair, February 23, 2012
Although we rarely find blood spots in eggs from supermarkets today, the custom to check every egg is still entrenched in observant Ashkenazi homes.
The Talmud (Chullin 64b) prohibits eating an egg with a blood spot if it might have been fertilised. But most chickens in industrial farms never see a rooster. Any blood spots in their eggs are from tissue irregularities.