Why does Frodo see his sword glow red at Weathertop?

Question

[...] He shut his eyes and struggled for a while; but resistance became unbearable, and at last he slowly drew out the chain, and slipped the Ring on the forefinger of his left hand.

Immediately, though everything else remained as before, dim and dark, the shapes became terribly clear. He was able to see beneath their black wrappings. There were five tall figures: two standing on the lip of the dell, three advancing. In their white faces burned keen and merciless eyes; under their mantles were long grey robes; upon their grey hairs were helms of silver; in their haggard hands were swords of steel. Their eyes fell on him and pierced him, as they rushed towards him. Desperate, he drew his own sword, and it seemed to him that it flickered red, as if it was a firebrand. Two of the figures halted. The third was taller than the others: his hair was long and gleaming and on his helm was a crown. In one hand he held a long sword, and in the other a knife; both the knife and the hand that held it glowed with a pale light. He sprang forward and bore down on Frodo.

_{The Fellowship of the Ring, Book II, Chapter 11: A Knife in the Dark.}

As we see in the excerpt above, when Strider and the Hobbits are camping in Amon Sûl on their way to Imladris, Frodo puts on the One Ring and enters the Unseen World, enabling him to see the real form of the Nazgûl.

Everything seems pretty straightforward and concordant with what happens when an individual enters the Unseen World (e.g., after being stabbed with a Morgul-knife, when Frodo is dying on the East Road, the description of the Nazgûl is identical).

My main issue with this passage is the part where "[Frodo] drew his own sword, and it seemed to him that it flickered red, as if it was a firebrand."

EDIT: As pointed by @A. Giesbrecht, Frodo was using a Barrow-blade given to him (and the other hobbits) by Tom Bombadil after rescuing them in the Barrow-downs.

Sting, as described in Tyler's The Complete Tolkien Companion,

[...] gleamed with a cold blue light if any servants of the Enemy where nigh at hand.

_{The Complete Tolkien Companion (Second Edition).}

However, nothing is mentioned about Sting having a red flickering; I haven't found anything on Foster's Complete Guide to Middle-earth or The Letters of J.R.R. Tolkien either.

Is there any explanation of this red-flickering phenomenon in any of Professor's Tolkien's texts?

Answer 1

Frodo was not yet using Sting at Weathertop. He received Sting from Bilbo in Rivendell, just a few chapters on. At Weathertop, he was using one of the swords rescued from a barrow & given to the hobbits by Tom Bombadil.

We know that the Barrow-blades were made to fight the Witch-king of Angmar, enchanted to counteract Morgul spells. (If I recall correctly, this is one reason why when Merry stabbed the King of the Nazgûl during the Battle of the Pelennor Fields, he actually did some damage.) I suspect that the Ring allowed Frodo to see the power in the blade.

Answer 2

I can't say why, but this was not the only object the Ring had such an effect on. This is a description of Isildur putting on the Ring just before he died, from "The Disaster of the Gladden Fields" chapter of Unfinished Tales:

Isildur turned west, and drawing up the Ring that hung in a wallet from a fine chain about his neck, he set it upon his finger with a cry of pain, and was never seen again by any eye upon Middle-Earth. But the Elendilmir of the West could not be quenched, and suddenly it blazed forth red and wrathful as a burning star. Men and Orcs gave way in fear; and Isildur, drawing a hood over his head, vanished into the night.

The Elendilmir being the gem worn around the forehead as he closest thing to a crown of Arnor. Though an inferior replacement was worn most of the time due to Isildur's being lost until long after the kingdom fell. The end of the quote would suggest it wasn't just Frodo seeing it that way, but the Ring changing the color of the light it gave off.