David Lee Moreno comments

I do agree that it is important to know your blood type in that it will help out in many instances. I do not agree with the way the Japanese do it, however. The person that said that it is traditional, get it together. This is more of a recent phenomenon than a years old tradition. This is not a tradition, this is another form of elitism that the Japanese are so fond of but don't admit. I do not say this with contempt, but rather out of observation.

People can and will have major points in their lives affected by their blood type! This is not a myth, this is the true hard FACT. Some people may try to convince you that it is something that the Japanese don't really take seriously, when in fact, most of them do. Everyone of them will at least think about your blood type, if not take a guess. And if they guess wrong? They knew the truth all along, and were just toying with you, of course. I've been at the first enkai of the year, where they introduce the noobs, and have watched conversations turn frigid because of blood typing(not that the alcohol helps things).

This is not to say that ALL Japanese people believe in it, but it is part of their lives whether they like it or not, and therefore everyone must know. So, as a foreigner, when you got a Japanese person that you know doesn't take it seriously, it is okay to share. You need to be 100% sure, though, as it can cause awkwardness in your friendship if they take it even half way serious.

Now, this idea of grabbing an idea and holding onto it even though there is nothing there is not unique to the Japanese. Every culture has something that they prescribe to that doesn't make sense. So, yeah, call it bupkis, but don't hate. In summary, if you know someone is not serious about it, share, but if you are unsure, refuse to tell.