If blood types show up in apes (as I'm pretty sure they do) then they probably all existed too far back to even matter which 'came first'--there likely were no real humans yet.

It seems like the blood type-diet connections just reflect how humans (other primates too?) dealt with the general 'feeding modes' found throughout nature: Herbivore=Agriculturalist, predator=hunter, etc.  Humans, adaptable creatures that we are, have evolved to be (as a whole) generalists that can make do in different environs and not get 'stuck' like other animals.  Blood type must capture this to some degree.  Ex: a random group finds sustainability in hunting deer; the O types have more of an advantage and soon dominate the population; other blood types, though still present, don't fare as well.