In a weird way, it is possible for Boromir to have "less true" blood than his brother. I remember, oh, so very long ago, the introduction to genetics we were given in biology class back in high school. Using a simpler genetic structure, the teacher showed how certain traits pass from parents to children, and how, depending on the parents' own genetic structure, it's possible for one child to show all the dominant genes while another child shows all the recessive. If the characteristics of Numenorean blood to which Gandalf refers are carried in the genes, it's possible (depending on the genes of Denethor and Finduilas and the ancestors of both) for Faramir to have those traits inherent to pure Numenorean blood, and for Boromir to have more of those inherent to their non-Numenorean ancestors. Just thinking along rather peculiar lines.
That said, I do tend to think Gandalf was speaking metaphorically rather than literally, but one never knows.
