Blood is a powerful symbolism, and sometimes even has mystical powers. The Blood Oath makes use of this to make a commitment that can't be broken. (Or if it is, it's crossing the Moral Event Horizon)
Bob has a secret he needs to get off his chest, but doesn't want it to be made public. Alternatively, he needs to be certain Al will do something for him. Either way, this is so serious that it [...] [tvtropes.org]