At 2:15:35 you will see the reference:
The blood types for the two main characters (i.e. Sean Archer and Castor Troy) are mentioned in the movie, and at a subtle level they reflect their antagonistic nature. Archer’s Blood type is O negative, while Troy’s is AB positive, these two blood types are as different as you can get. O negative- the universal donor reflects Archer’s role as a policeman dedicated to serving the community, while Troy’s AB positive – the universal recipient suggests someone who takes from society without giving anything back.
Source: IMDB