The first Mishna in the fifth perek of Massekhet Nazir deals with mistakes made when declaring something to be a sacrifice. The example brought – about which Beit Shammai and Beit Hillel differ – is a case where a man declares, “Shor shahor she-yetze mi-beiti rishon, harei hu hekdesh” – that he is prepared to sacrifice “the black ox , which will come out of my house first.” If a white ox comes out first, Beit Shammai believes that it becomes hekdesh (consecrated), while Beit Hillel rules that it does not.
This disagreement leads the Gemara to discuss the advantages and disadvantages connected with white animals and black animals. Rav Ḥisda teaches that “black among white is a deficiency” and that at the same time “white among black is a deficiency.”
These statements are understood by the commentaries as follows. According to The Meiri, “black among white is a deficiency” refers to a situation where there is a row of white animals, and black ones appear among them. In such a case, the black animals lower the value of the group, and the potential purchaser may choose to look elsewhere to purchase his animals. The case of time “white among black is a deficiency” refers to a white spot on a black animal. If an animal is from a particular breed that is ordinarily black, rather than pointing to a mixed breed, a white spot may very well indicate an injury or some other deficiency in the animal. Sometimes such a spot can show that the animal suffered a serious blow to that particular part of the body that caused the hair roots to lose their natural pigment. Even if there are no other physical manifestations of this blow, it is possible that internal damage was done to the animal, as well.