Keritot, Chapter Five, Mishnah One
The Torah prohibits the consumption of blood. One who eats blood is liable for karet if done intentionally, and that is why this halakhah is found in tractate Keritot. If done unwittingly, he is liable for a hatat.
1) If one ate blood of a slaughtered beast, a wild animal or a bird, either clean or unclean, or blood of an animal stabbed in his throat or neck, or of the blood of an animal slaughtered by having his throat ripped, or of the blood of the arteries whereby life-force escapes, he is liable.
2) But [if he ate] the blood of the spleen or of the heart, or blood found in eggs, or blood of fish, or of locusts, or secondary blood, he is not liable.
3) Rabbi Judah says: he is liable for secondary blood.
Section one: This is a list of all of the kinds of blood for which one is liable for a hatat, as long as he eats an olives worth of it. The blood does not have to come from a clean (kosher) animal, nor does it have to be the blood of an animal that is slaughtered in a kosher fashion. As long as the blood is the blood whose loss causes the death of the animal, one who eats it is liable. The idea that one is liable only for this type of blood seems to come from Leviticus 17:14, which refers to blood as the life-force. The rabbis deduce from here that one is liable only for blood which is the life-force, and not for other types of blood.
Section two: This is a list of either internal organs, taken out of the animal after it is dead, or blood found in other things, such as eggs, fish, or locusts, or blood that comes out after the animal is dead (secondary blood). While some of these are prohibited, they are only prohibited by the rabbis and not by the Torah; one who eats them is not liable to bring a hatat. Note that Leviticus 7:26 specifically states that animal (beast) and bird blood is prohibited. Since it doesnt list fish or locusts, the rabbis deduce that fish and locust blood is not prohibited.
Section three: Rabbi Judah holds that one is liable for eating secondary blood.