Reb Yidd123
The prohibition of eating blood is only animal blood from the Torah. The rabbis prohibited all blood because it could be mistaken for real blood. So for example if your gums are bleeding. If you bit a piece of bread and there is blood on the bread you must remove the part that has the blood although you can swallow the blood in your mouth.
It is the same as Copepods as long as they are in the water they are permitted but once they are on land they are prohibited. Here too the vampire sucks the blood directly from the person it would be permitted. If the vampire was to open a bottle of blood it would be prohibited.