Blood found in eggs is forbidden for consumption, for this blood indicates the beginning of the embryotic development of the chick and this chick has the halachic status of “fowl” whose blood is forbidden for consumption by Torah law; thus, the opinion of the Rosh and Tosafot is that blood found in eggs is likewise forbidden to be eaten by Torah law. Nevertheless, according to the Rambam, blood found in eggs is only forbidden to be eaten by virtue of a rabbinic prohibition, for although this blood indicates the beginning of the chick’s embryotic development, since the chick’s development is not yet complete, it does not retain the status of “fowl” and its blood is not forbidden by Torah law; rather, there is merely a rabbinic decree banning it due to its similarity to blood of an actual bird.
There are instances where the entire egg becomes forbidden for consumption as a result of this blood and there are other instances where the entire egg does not become forbidden and one must only remove the blood and the rest of the egg is permissible, as follows:
If one finds blood (even a drop) in the yolk of the egg, the entire egg is forbidden. However, if the blood is found in the egg-white, one may remove the blood and the rest of the egg is permissible for consumption. This applies according to Maran Ha’Shulchan Aruch. However, according to the Rama, if blood is found in any part of the egg, the entire egg is forbidden for consumption. The Sephardic custom is to rule leniently on this matter in accordance with the ruling of Maran.
Although blood found in eggs is forbidden for consumption, there is no obligation to check every egg before eating it since we rely on the fact that most eggs do not contain blood and according to the letter of the law, one may rely on the majority. Only if one is frying eggs in a pan or breaking open eggs for baking and the like, it is customary to check the eggs for blood (in a glass or any other clear vessel), for in this case, it is quite easy to check the eggs for blood since they are being opened anyway. However, when one wishes to hard boil eggs in water or place eggs in a Chulent and the like, although after cooking them it will be impossible to tell if there was blood in them or not, it is indeed permissible to do so without checking them, for most eggs do not contain blood and we follow the majority. The same applies to one who wishes to swallow a whole egg, uncooked, that one need not check it first.
Nevertheless, the Kenesset Ha’Gedolah writes that there are some who are stringent not to eat eggs unless they have been checked for blood first; even if they wish to hard-boil the egg, they make a small hole in the shell, pour out the contents of the egg into a cup, and after ascertaining that the egg is clean of blood, pour the contents back into the shell, close up the hole, and then proceed to hard-boil it. However, halachically speaking, there is truly no room for this stringency, for Rabbeinu Chaim Vital writes that he has seen his mentor, the saintly Ari z”l, eating a hard-boiled egg without first checking if there was blood inside. He proceeds to bring sources from various places in the Talmud that there is no need to be concerned about the more stringent opinion on this matter. Maran Ha’Chida quotes all of this in his Birkei Yosef and concludes, “Now that the holy teachings of our rabbi, the Ari z”l, have been revealed, there is no need to be concerned with the aforementioned stringency some individuals follow.”