Actually, he says that it runs high in both Denethor and Faramir, but not so much in Boromir. Maybe Boromir took after their mom, Finduilas of Dol Amroth.... But of Denethor it is said, "in him the blood of Númenor ran nearly true."
Boromir is a man more like Éomer - "we are men of action; lies do not become us." He's a warrior, and a proud one....but that's about it. In some ways, he is like Eärnur, last king of Gondor.
Faramir and Denethor are both more "subtle."
They are the ones who can read men's hearts. They can be politicians. They....are "high" men.
This is (in part) why the family dynamics are so interesting. Denethor loved his heir, Boromir, even though his other son, Faramir, was more like him in temperment (though not in mood). Faramir has the same subtlety as Denethor, but he is much milder, and less proud. He is moved to pity rather than scorn by what he sees of other men. Boromir, on the other hand, has the same mood as his father, which is why they are able to understand each other so well.
(This is in the Appendices, I must admit. But I do think that he used these descriptions to craft the dialogue. All the references to 'wizard's pupil' speak of this underlying truth.)
To speak of two brothers having different "blood," it must be understood that blood is being used somewhat metaphorically. It is being used of traits of the Numenoreans that may or may not be passed down. So, Denethor, who is not of the line of Elros (but still of High Númenórean blood) isn't having his blood analyzed for similarities to the Númenórean kings. He's...having his character, bearing, and personality traits analyzed, and then being told, "yep, you're like them."
Blood means inheritence, but is not limited to biological inheritence (genetics). It is the broader scope of anything that could be passed down.
Aragorn looks an awful lot like Elendil, and reminds Elrond of him. It is unlikely that he has that much in common with him, geneticly speaking, because the genes have passed through a bunch of generations. But....there is still a family resemblence, something in who he is as a person that strikes Elrond. (The resemblence of Elendur, son of Isildur, to Elendil is much more likely to be tied to genetics - Elendil is his grandfather.)
My brothers and sisters and I all have blue eyes, and anyone would point to this as signs of our "Irish blood." But they'd say the same thing of my mother's temper (not just her red hair). Or use it to explain why so many of my relatives have a drinking problem. Or why I like potatoes so much. I mean....it's broader than genes. Blood also incorporates culture. (Or stereotypes, as the case may be.) When Faramir speaks of being mixed with lesser men, he doesn't just mean that the grey-eye trait is going to die out as they inter-marry...he means that their culture is going to grow closer to that of the people who surround them. They are becoming more like the Rohirrim. It's not necessarily a bad thing, but it is a change, a losing who they were....
Pssst,
Tom - I know very well how lengthy my posts are! There was a reason I paused before calling Faramir names
.