Blood brotherhood is a bond of trust between two or more people by a blood pact ritual involving physical acts such as rubbing bloody wounds together or consuming each other's blood with food or drink.
This is also known as body-substance siblinghood, a term derived from eastern and western central Africa, however, this bond does not necessarily have the sharing of blood.
Those who practice blood-siblinghood deem it to be a deeper, more meaningful bond than biological siblinghood. Interestingly, blood-siblings refer to each other in terms of absolute equality whereas biological siblings of the same society do not.
Source:
Thomas Barfield, Dictionary of Anthropology, 1997, Blackwell Publishers