"The stereotypes must have arisen strictly from the mere existence of the labels."
Actually, there is an interesting history to where these (completely bogus) stereotypes came about. Wikipedia has an article on this that explains the historic roots of this fad in racism and politics. For example, Japan had a political conflict with Taiwan, and someone noticed that Taiwanese people are predominantly type O. He therefore argued that "Taiwanese rebelliousness" was genetically determined and that blood type was a marker of this. Clearly this is pattern recognition gone wild, but there was an actual political agenda motivating this false pattern recognition. Therefore, the blood type fad did not originate "strictly from the mere existence of the labels." Of course it is possible that the fad has persisted long after the original political agendas have been forgotten (although I am speculating here) and this meme has taken on a life of its own so to speak.