The concept of cum being described as blood milk is the human body uses blood to make sperm/cum; this is how a college coed explained it to me. It's a term being used on college campuses that's all; it has nothing to do with blood/bleeding, biting nor pain.

I am NOT into biting, blood nor pain.

Some college students came up with the slang lingo of calling cum, blood milk, to kind of shock people and/or to be able to say they coined a term.