I agree with the general consensus here: I don't think one's blood type affects their personality really. Blood type could affect your physical/metabolic processes, though, which indirectly could affect your mood. (i.e. If you have a high metabolism but don't snack enough throughout the day, you could experience constant hunger and thus act slightly crankier than you usually would.)

Still, it is a neat little detail anime/manga creators add to their characters. :)