I wonder if they have more than just human blood there. Drinking blood shouldn't be a problem, it should be healthy even, but I would still hesitate when it came to human blood. Not a fan of cannibalism. Just say you're on a special diet and can only drink pig's blood, or cow's blood. Perhaps it needs to be specially prepared so you have to eat Blood Sausage or Cow Hearts. These two are rather tasty and it should be an easy enough excuse.