You shall not eat any blood… (Leviticus 7:26)
If you want unambiguous, this mitzvah is about as straightforward as they come. In its entirety, the verse reads, “You shall not eat any blood – of birds or of animals – wherever you may live.” There’s simply no mistaking that.
This mitzvah is repeated in several places and Leviticus 17:11 attaches a reason to this prohibition: the life force of a living thing is in the blood. Therefore, God has designated the blood for use in effecting atonement through sacrifices.
The Sefer HaChinuch reiterates the concept of “you are what you eat.” He says that we are forbidden from eating different things so that we do not internalize spiritual qualities associated with these items. He associates the blood of an animal with the trait or cruelty. Since the life force is in the blood, eating it is just wrong.
This verse says quite clearly that the prohibition applies to birds and animals. It does not apply to fish, locusts or even people. (That’s not to say that drinking human blood is permitted, it’s just not covered by this prohibition. The kashrus status of humans is complicated. Human milk, for example, is both kosher and parve!) In any event, human blood is prohibited rabbinically – see Talmud Kerisos 21b – though if one’s gums bleed while flossing, that’s not problematic from a halachic standpoint. (It may be problematic from a dental hygiene standpoint, but that’s beyond our scope.)
The prohibition on eating blood is the reason that kosher meat is soaked and salted; the process draws out the blood. This process is insufficient for kashering liver, given the large amount of blood it contains, so liver must be broiled. Additionally, the practice is to check eggs for blood spots prior to use. (One should consult one’s rabbi for details on this halacha given the realities of modern commercial egg production.)
“Lifeblood,” which comes out at the time of an animal’s slaughtering, carries the penalty of kareis (spiritual excision) for its consumption; the blood in the limbs and certain organs does not, though it is still prohibited. (See Talmud Kerisos 22a.)
This mitzvah applies to both men and women in all times and places. It is discussed in the Talmud in the first and fifth chapters of tractate Kerisos as well as in the eighth chapter of tractate Chulin. It is codified in the Shulchan Aruch in Yoreh Deah 65. This mitzvah is #184 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #89 of the 194 positive mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.