This is one of the softest things I've read.
The premise of this chapter is obvious, but getting it to feel like this is a whole 'nother basket of fish.
@minmanga:
Though I'm not sure of the origins of it (it obviously can't be that old a convention), blood-types in Japan are treated a lot like astrology signs here, in that people will (with varying degrees of joking-ness) expect certain "personality traits" from certain blood types (as with astrology, for those who do believe in it this works because the traits described are kind of vague and universal). I think kitschy fortune-telling likewise often incorporates them too but don't quote me on that.
So it's like saying "Luka, 9 months old, Scorpio", in essence and tone.
Which, just as with listing an astrology sign, isn't to say that the author must believe in such things—it can just be being childish and cute, too.