The Bread of Life discourse reaches its most startling language when Jesus says that unless people eat the flesh of the Son of Man and drink his blood, they have no life in them. The imagery was not culturally neutral.
Israel's law repeatedly prohibited consuming blood because the life of the creature was associated with its blood.
Leviticus 17 and Deuteronomy 12 prohibit eating blood and explain that the life is in the blood. Jesus' wording therefore could not have sounded like an ordinary invitation to consume animal blood. The discourse develops earlier bread imagery into intensely personal language: his flesh is given for the life of the world, and true life is found in receiving him.
Christian traditions differ over how directly John 6 should be related to the Eucharist. The Jewish blood prohibition remains essential background regardless of that later sacramental debate.
The prohibition explains why the saying has such force. Jesus chooses imagery that no observant Jewish hearer could treat casually.
John's point is not permission for literal blood consumption. The discourse insists that eternal life cannot be separated from Jesus himself and ultimately from the self-giving embodied in his flesh and blood.