The blood type diet is or was a theory that we should personalise what we are eating based on our blood type. There are eight main blood types: A positive, A negative, B positive, B negative, AB positive, AB negative, O positive and O negative, and according to the theory each blood type responds differently to certain foods and hence it is better for our health to do so.
This theory came from a naturopath and the logic is based on a number of complete assumptions, which on further review seem to me a complete misunderstanding of basic biology and evolution. The basis of the theory is that the O blood type is the most ancient blood type and hence should be paleo, then A group should be vegetarian because they were farmers and B group have dairy because they were all nomads in the past. This sounds pretty silly to be lumping people into such categories and they have not based this on any real research or evidence. It is just a theory that someone came up with. But even the part about the ancient O blood type is not true as far as we know as the A blood type is the most ancient. So even at its core the theory does not hold up.
There is no evidence for benefit in weight loss and there is actually no evidence of it in any condition. Almost no studies have been done on it and those that have been done show no benefit. It is a classic fad diet that someone has pushed to make lots of money. It is simply ideology and unethical marketing.
The reason some people support it is the same reason people support lots of diets, which is their personal experience. See the blood type diet tells you to overall eat well regardless of the specific recommendations which mean more real food and less processed food. And guess what- that is what every diet recommends, and anyone who goes from eating a poor diet to a diet high in whole foods and less processed and packaged foods – is going to feel better. So it is not because of the blood type, it is because of the general changes made. But people are tricked and persuaded by someone selling them a belief and a product that it was “their product” that helped them. It wasn’t, it was just common sense changes that I have discussed many many times above.