Written by Rabbis Eli Gersten, Yaakov Luban and Moshe Zywica of the Orthodox Union
The rule is that if a fertilized egg develops to the point that a blood spot appears on the yolk, the entire egg becomes forbidden!
However, today’s commercially sold eggs are assumed to be non-fertilized eggs. If one finds a blood spot on a non-fertilized egg, simply remove the blood spot and use the rest of the egg.
However, Rav Moshe Feinstein, zt”l, maintains (Iggerot Moshe Y.D. I:36) that since eggs are relatively inexpensive, the common custom is still to throw away the entire egg.
If the egg is already mixed with other eggs or food and is difficult to remove, one can simply scoop away the blood spot. If the blood spot itself is already mixed in, and cannot be removed, the blood spot is batel (nullified) and the food may be eaten.