Temptation of Boromir

[Raynor]

Is it because he is Isildur's heir, and had a "right" to the ring or is it because he is of noble blood, or what?

I think it is his moral nobility that protected him; the ring has only one master, as noted by Gandalf

Originally Posted by Council of Elrond, FotR

We cannot use the Ruling Ring. That we now know too well. It belongs to Sauron and was made by him alone, and is altogether evil.

and Tolkien himself

Originally Posted by Letter #246

Sauron would not have feared the Ring! It was his own and under his will. Even from afar he had an effect upon it, to make it work for its return to himself.