blood on blood ?

Hi, everyone

Do anyone knows what “blood on blood” means ?.
I see it in a song. Could you help me ?

Hi Cheng, it probably refers to a ritual that is often performed by children or in cults, in order to become blood brothers. Two people prick themselves or give themselves small cuts, let small amounts of blood issue from their wounds, and mix their blood. They could, as an alternative, press both bleeding wounds together to mix the blood. Or they might write their names in their own blood.