purely hypothetically, could a human live on nothing but animal blood?

I actually asked a medical doctor about this one and the answer is no. blood doesn't contain the nutrients we need to survive in the quantities it takes to keep us alive, it messes up our digestive systems (results in the most horrific smelling excrement and gas you can imagine) Blood is not a food capable of sustaining us long term.

Also, you will likely get iron poisoning.

Iron poisoning is rather unlikely as the concentration is simply not high enough. As already mentioned, blood tends to be very low concentrated on other nutrients as well. Much data can be found in studies about vampire bats, btw.

(results in the most horrific smelling excrement and gas you can imagine)

 

This is helpful in spotting vampires.