The blood of Faramir but not Boromir?

Well, Faramir and Denethor are definitely alike. But in the Appendices it does say that Denethor and Boromir were alike in looks and pride, while Faramir was liken to Denethor in insight (ability to read the hearts of men) and looks. And, while we don't know much of Finduilas, we do know that the line of the Princes of Dol Amroth were of Numenorean and Elvish descent. So she could have been a (Ww) for all we know, and though the chart would show that the kids would all be (Ww), and since there's always the possibility of an oddity, it could have happened, as long as the gene is still there.

I always wondered how Denethor had the blood of Numenor, when he obviously wanted the ring, etc.

Well, the Stewards do descend of Elendil, not in direct descent (meaning father-father), but one of the Stewards definitely descend of Elendil and Anarion through a female. Which might explain Boromir's confusion at why the Stewards weren't kings. It would also explain that Gondorian custom differed slightly from Numenorean custom. Oh yeah, and Isildur was Numenorean (and one of the faithful) and he took the Ring. Boromir probably inherited the traits of his ancestors from the other Houses (meaning from the 3 Main Houses, one of the lesser ones).

Faramir refused the Ring but then he didn't have it working on him for three or four months, undermining his resistance.

I agree with Lyra Greenleaf on this. Boromir wanted it from the start. And, to add, Boromir was with the rest of the company of the Fellowship, which proves a bit of an odd situation if he were to take the Ring. Meanwhile, when Frodo and Sam are in Henneth-Annun with Faramir, the younger brother has about two hundred men at his disposal, and he had them cornered after the meal. Of course, he purposely intimidated/made fun of them by standing up and saying "Hah!...etc, etc," but he still rejected the Ring. If not in oath, then by nature (recall how he says: "Even if I were one to desire such a thing...").