The eucharistic blood does not run afoul of Acts 15:20 for the same reason that the eucharistic body is not cannibalism, even though it is the eating of the body of Christ. There are two answers to this question here, but I don't like either one.
Transubstantiation is a metaphysical reality, not a physical reality. If you know about metaphysics, then the substance changes, but the accidents don't. If you're not into metaphysics, then it's not the appearance of the bread and wine that changes, but the significance of the elements.
We're not eating a human body that we've just killed and boiled in a pot. We're eating the second person of the living deity that has just entered into bread and wine, ordinary-looking food, so that He might enter us and commune with us. In the same way, the second person of that very same deity once entered into a physical human body and lived a life on earth and died a horrible death, but then rose from the dead and ascended back into heaven. His sacrifice, once and for all, saves us and we participate in that sacrifice by taking communion.
By analogy, in the Old Testament the Jews had a communion sacrifice in which a lamb was killed, part was burned on the altar, and the rest was consumed by the family. The significance here was that God was sharing a meal with the family. The mutton was no longer ordinary meat, but had been made holy by the sacrifice. God was present because He resided in the temple, and because He was invited to the family's meal by the sacrifice.