Converging into the Transcendent
In An Indomitable Will, I wrote on the way the green lantern story is in many ways the modern story, it's the philosophy of secular humanism in beautiful myth. We also see that it's a flawed philosophy. I don't know if this was intentional on the part of the authors, but the undead Abin Sur's statement to Jordan in Green Lantern #47 (2009) is very damning for not only Jordan but the entire Green lantern Corps:
Abin Sur: I was obsessed with the Future. You ignore it. We could not be more different, yet the ring chose you. Why?
You may have found purpose in the corps like I did, but you have no path. You take flight without Destination.
Hal Jordan is the greatest green lantern, and the reason is the above quote. The green lanterns use Will power for Will power's sake, their moral mission has no particular direction, it's like Guy Gardner says; the guardians make things up as they go. No one embodies this more than Hal Jordan, whom the Flash describes as always running half cocked into anything.
The Black lanterns cannot be harmed by green rings alone because those rings feed right into the logic of the black rings, Will power for Will power's sake is an Oroboros, an absurdity, a form of death. The way the rings are destroyed reverses this. It is when the green light of will is combined with the other colors of the spectrum that they finally defeat the black rings of death.
The formerly unguided willpower now has purpose, purpose transcending itself, a "Telos", or end, whether it be Fear or Love. And the combination of all the colors into one white light of life is the ultimate Telos, as life IS Telos. The loss of an end condemns the means to death.
As long as the green lanterns are without the Telos that comes with union with the other parts of the spectrum, they will keep failing, and if they are the mythology of modernity, that tells you a lot about where we're headed as a civilization.