I think the idea is that the blood type alleles are not compatible with foods and ingestibles that the body hasn't evolved with.
For example, the h-g foods work extremely well for me because I'm type O, the oldest type, which was the only type during the time way back when the primary method of getting food was hunting and gathering. Most grains, potatoes, peanuts, and other foods are harmful to me. Almost all meats, many fish, and poultry are highly beneficial, along with leafy green vegetables, fruits, berries, and nuts. Eggs are OK once in a while, and dairy a little but not a lot.
Now, type A evolved with agriculture and is supposed to better with a vegetarian style diet; type B with dairy. AB has its beneficial and harmful foods, too.
Of course, some of the foods overlap; and I suspect that some people can do well on just about any diet. It's even said that some evolve spiritually, kind of like a yogi, to the point that they turn all harmful foods into neutrals.
All I know is that it works for me, to a tee... complete good health and no arthritis.