I have another route into this thread.
there is a subculture of therians... who beleive that the blood offered binds them to the one receiving. it's not so much about the blood itself as the act and symbology. ( yes i've participated numerous times. )
Another subculture of that one is the sharing of blood to bind relationships. sort of the same but this one is to create familial ties. Actual mixing of blood. Sometimes portrayed in hollywood movies by a Native American cutting his palm or wrist and holding it next to the one he wishes to make a "blood brother". This too i have participated in.