Longing for a reason to discriminate other people, Japan simply invents one.

[DetlefK]

4. It's not banter. Some Japanese do practice this kind of racism. From the same article:

In some cases, blood type becomes an excuse. In 2011, Japanese Reconstruction Minister Ryu Matsumoto was forced to resign because of callous comments about the areas devastated in the Tohoku earthquake and tsunami that year. In a post resignation conference, he took the opportunity to blame his behavior on his blood group: “My blood is type B, which means I can be irritable and impulsive, and my intentions don’t always come across. My wife called me earlier to point that out. I think I need to reflect on that.” He was politely pushed out by the administration for his gaffes.