[Boromir88]

Very fascinating stuff obloquy. You talk about the barrow-blade and as Aragorn says with other 'blades' that strike the Witch-King:

A foot above the lower hem there was a slash. 'This was the stroke of Frodo's sword,' he said. 'The only hurt that it did to his enemy, I fear; for it is unharmed, but all blades perish that pierce that dreadful King. More deadly to him was the name of Elbereth.'~Flight to the Ford

Aragorn makes the point that shouting 'Elbereth' was more deadly to the Witch-King. Also:

Escaping a wound that would have been as deadly to him as the Mordor -knife to Frodo (as was proved at the end), he withdrew and hid for a while, out of doubt and fear both of Aragorn and especially of Frodo. But fear of Sauron , and the forces of Sauron's will was the stronger....
But above all the timid and terrified Bearer had resisted him, had dared to strike at him with an enchanted swords made by his own enemies long ago for his destruction. Narrowly it had missed him. How he had come by it - save in the barrows of Cardolan. Then he was in some way mightier than the barrow wight; and he called on Elbereth, a name of terror to the nazgul. He was then in league with the High Elves of the Havens.~The Readers Companion; The Marquette Letter

As Raynor discussed in past threads that the Witch-King judged Frodo to be a powerful foe. Frodo's spiritual power I don't think was near the Witch-King's but Frodo did have

The Barrow-blade
Calling on Elbereth (a name that one could say has spiritual power):

Out of the deeps of Ea she came to the aid of Manwe; for Melkor she knew from before the making of the Music and rejected him, and he hated her, and feared her more than all others whom Eru made...
Elbereth they name her, and they call upon her name out of the shadows of Middle-earth, and uplift it in song at the rising of the stars.~The Silmarillion: Valaquenta

So Frodo did possess dangerous and spiritually powerful weapons that caused the Witch-King to flee...and eventually the Barrow-blade overthrew the Witch-King's power.

We might also extend this to the Nazgul. Tolkien said that their power was primarily in the fear that they projected, so if a character such as Eowyn has in her spirit the power to resist that fear, she may be able to overcome the power of the Nazgul and thus penetrate that barrier created by it.~obloquy

Another interesting point; as when Frodo struck back at the Witch-King, part of the startlement was that Frodo actually summoned the courage to strike him; as the Marquette Letter points out 'the Bearer had resisted him; had dared to strike at him.' It defintely was a shock to the Witch-King that the power of his fear was overcome by Frodo. And I think we can say the same for Eowyn...as the Witch-King is described threateningly:

Out of the wreck rose the Black Rider, tall and threatening, towering above her...

But she's able to overcome this and drive her sword between 'crown and mantle.'