What kind of things do you think a Mage of Blood would be able to do? Obviously, there's creating relationships, but how could that be used to do damage, whether emotional or physical?

Knowing Blood likely means they know exactly where to cut to make their target bleed out with a single strike. Blood Majyyks may permit them to also manipulate Blood a-la Red Miles or just general Blood Magic, not to say anything of just being able to manipulate friendships and get their way with that.