what if it was because blood inside you takes up a blue color, ya know, and they drink so much blood, which happens to be red outside off the body, that it would intertwine with their blood stream, and there be so much of it that it shows through their skin, and what color do red and blue make when mixed…? But it also could just be they drink so much blood it goes through their skin and is a purple color, but i like my theory better, it sounds cooler.