I did a bit of research on the blood type diet fad when someone on another forum mentioned it; they swore by it no matter what ridiculous info was thrown up in relation to it.
Most people end up being considered in the central band (as I'll call it) and therefore end up being given the exact same diet; but, the weird thing is that a questionnaire is used to assess which diet is right for you, not just your blood group...that's where they lost me.
The questionnaire used also alters and you can have a more in-depth questionnaire or just use one of the many basic ones that are floating about on the internet. But, when someone starts basing your diet on how social you are, how introvert/extrovert, which type of sports you prefer, whether you're emotional and prone to outbursts etc...that just takes it out of the diet realm and into the twilight zone for me...
So, verdict, rubbish! (IMO, of course)