This reminded people of those pagan feast rituals where a sacrifice of an animal was offered on the altar in worship of a god and some of the meat of the sacrifice would be given to a family to enjoy a religious feast, and while they ate the meat offered for sacrifice their belief was that they were actually ingesting the god they were worshipping, and the god was coming inside of them and becoming a part of them. Eat Jesus' flesh. Drink Jesus' blood. The devout Jews who listened to Jesus were repulsed by these words. Blood is sacred to the Jews.
Blood carries life.
If a person gets a wound and the blood flows out of them their life flows away as well. Blood stands for life. And life belongs to God. Therefore, blood belongs to God who gave it. A good Jew would not eat any meat unless it has been completely drained of blood.