I think there is some correlation between blood type and dietary needs, but remember, correlation does not prove causation. Blood type, like skin hair or eye color, are all traits determined by phenotype (the part of us that makes us individuals rather than human clones
). Thing is, phenotype traits can be mixed. When a black parent and white parent has a child, they don't get an
absolutely black child or
absolutely white child, they get some blend of both. So, even if your blood type happens to be "o", which you inherited from one ancestor, and o blood types are what the hunter-gatherers had, you
personally might have inherited a more modern ("evolved") metabolism from a
different ancestor. So you see, there is very little evidence to support the theory that blood type is absolutely linked to metabolic plasticity (i.e. ability to digest dairy, refined grains, etc) since blood type and the systems which make up metabolic systems are not determined by the same genes, and the traits can be mixed.
Plus, there are many rare blood types out there, especially among enclaves of people who decend from hunter-gatherers... these rare blood types circulated because of the close inbreeding. One example is the dieago blood type, found only in some NA's. If anything, having a blood type like AB shows evidence that your system is highly adaptable.