Keritot 21 - Why Is Consuming Blood Prohibited
The Torah said, " Do not eat any blood, whether from a mammal or a bird, no matter where you may live ." However, that only concerns the blood of animals, who walk on four legs. How do we know the law regarding blood of those who walk on two legs (humans), the blood of eggs (either chicken eggs or testicles), and blood of creepy crawlies (such as rats)? After all, humans are fundamentally different from animals in that they transmit severe ritual purity when dead, while animal meat can transmit only the light impurity of foods.
But is that so? What if one plans to eat a piece of human flesh, doesn't it receive food impurity? - No, it has a more severe impurity of a dead body! But what if one took a piece less than an olive volume and added it to other food, so that it does not have the impurity of a dead body? - His intention accomplishes nothing: since people in general find human flesh repulsive, his intention to eat it does not give it the status of food.
Blood of grasshoppers and fish is completely permitted, because they themselves do not require kosher slaughter. However, if it is collected in a vessel, it needs to have a sign, such as fish scales, so that people would not mistake it for animal blood.
Art: Joachim Beuckelaer - The Fish Market