It was one of those faddy things endorsed by celebrities, it was alleged that Sporty Spice was once following it. The idea was that according to your blood group you ate certain foods, i.e dairy products , meat, or veg. It sounds ridiculous and is probably why it never really caught on.
blood group diet, not really a diet. it is about sensible eating for your particular blood type. nothing really faddy about it at all. possible explanatons to why certain bloodgroups are lactose intolerant or gluten intolerent.