Actually, milk has pus in it, not blood. The cows are milked by machines that have one udder in each "claw", and these "claws" are constantly moving up and down to extract milk when the "farmers" decide it's milking time. The constant movement forms blisters on the cows udders because of the constant udder-irritation. These blisters are not noticed and are not given time to heal properly because that would waste the "farmers" money, so the cow is continuously milked and the blisters pop, pouring the pus into the milk. I'm sure after a long enough time the cow's blood could enter the milk through more irritation, but that's more noticeable than milk-coloured pus. Also, chocolate milk doesn't come from chocolate cows, so how would chocolate milk have blood and not regular milk? Because it's dark and the blood is overwhelmed by the brown colour? It's white first, someone would notice.
Vegan friends rawk, don't they?
/rant for the day.