In the text, blood is a metaphor for the penchant for violence shared by Paul and Michel. Not only do Michel and Paul both commit acts of violence, their shared blood suggests that this tendency might be hereditary, a social disconnect passed down from one generation to the other compelling Paul and his son to imagine and act upon terribly violent thoughts.

Blood appears when characters lose their grip on their violent tenancies. When Paul is talking to someone he dislikes, he imagines punching them in the face. But his imagination goes further. He describes how the blood spurts and the teeth break, detailing the way in which blood becomes evidence of his strength and power. But most of these thoughts remain inside Paul’s head. On a number of occasions, they manifest as reality. If Paul’s actions are believable, he hospitalizes Michel’s principal and attacks Serge with a pan. On both occasions, imagined blood becomes real blood. The transition from imaginary to real—the way in which violent ideations become real attacks—appears through blood. Actual blood appearing signifies Paul’s most violent thoughts becoming reality.

This is a trait Paul has in common with his son.