Food woo is potent stuff, and Peter D’Adamo is one of the most influential. Now, there is some evidence that his suggestions are, indeed, partially founded on Eastern (more closely Japanese) superstition (many remedies that claim such background are not), though it must, for obvious reasons, be a relatively new kind of superstition. The Eastern connection does, of course, not make his crackpottery more likely to be correct.
So what’s D’Adamo’s thing? D'Adamo's schtick is the Blood Type Diet (a.k.a. blood type astrology), in which the members of each of the four main blood type groups are assigned their own regimen of foods to consume and avoid. D’Adamo’s book, Eat Right 4 Your Type, was on the bestseller lists for a long time, despite (or, the cynical may suspect, because of) a complete lack of scientific evidence to support the central idea, namely that our body's immune system reacts differently to different foods, and tht those reactions are determined by the various antigens specific to the blood types. Apparently he is putting a lot of effort in capturing the Japanese market, as seen from his website.
Throughout his books D'Adamo cites the works of various biochemists and glycologists who have actually researched blood groups, implying that their research supports his ideas. The consensus among dieticians, physicians, and scientists, however, is that the theory is unsupported by scientific evidence. Remember that the Galileo gambit is a fallacy, if you were ever attempted to go down that route.
The wikipedia article is rather overly evenhanded. A more neutral and comprehensive evaluation is found here. (It might, by the way, look like the idea was dreamed up by his father, a naturopath named James D’Adamo – it is not quite clear).
His self-written biography is here.
Diagnosis: Frighteningly common type of crackpot who confuses anecdotal evidence and confirmation bias with science. Some of his advice may actually be harmful, so he must be considered moderately dangerous.
He is of course not alone in peddling this kind of woo. Obstetrician-Gynecologist Steven M. Weissberg, M.D., and Joseph Christiano, a personal fitness trainer, have co-authored a book making similar claims – the slogan being “You are what you eat, but you should ‘EAT WHAT YOU ARE.’ This means each of us should eat the optimal diet compatible with our blood type.” As with D’Adamo’s work, theirs is backed up by a multitude of anecdotes and tenuous correlations.