I always thought the whole concept is bogus crap to put it mildly.
They fail to take in to account metabolic differences to begin with. Let's say for example they say chocolate is great for type A people. So we have 3 people with type A blood. One is healthy, one is diabetic, and one has chronic Candida issues. As we know the chocolate is not good for the last two individuals in particular.
People are too individualistic. We cannot group people in to simplistic groups and say "X' is good for all people within this group. That is as stupid as the people claiming one cause for all diseases.
Another thing to keep in mind is that what your body needs and will handle can change throughout life. For example when I used to race triathlons and did cross country bicycling I was living on around 10,000 calories a day of carbohydrates. And I had all the energy in the world. I even road my bicycle, loaded with gear, once 200 miles one day and got up the next day and went the final 100 miles to my destination. This kind of stuff was normal for me. But if I started eating meat my energy levels would bottom out after about 3 days and I would be lucky if I could do 50 miles in a day. When I went back to the high carb diet I would be back to my normal energy levels. But now later in life I have had to change my diet. I no longer carbo load and now eat a more balanced diet, which includes meats. If I were to eat the way I used to eat I would not fit through the door. So my blood type never changed, but I still had to change what I ate to fit my body and lifestyle.
As another example what about people who can tolerate milk when they are younger but later develop a lactose intolerance? What about people who do fine on meats or other fatty sources until they have their gallbladders removed? Then there are the differences between men and women. Younger women in particular may benefit from red meats for the iron. But men can easily develop an iron overload. And the additional iron can be a problem for the same women when they reach menopause or in both women and men with hemochromotosis.
The whole eat for your blood type concept was nothing more than a very clever sales ploy. Not only did they sell the books to convince you of this garbage, but they also sold the foods and supplements based on the books claims.