A definitive answer is impossible, since all we have is what JRRT wrote and it's always possible to deny supernatural/magical effects as illusions or imaginative language -- as, indeed, in some cases they probably are. But throughout LotR (and the Silmarillion, but I will not go there for now) light/shining/fire/glowy effects are a common part of the manifestation of magic/supernatural. For example:
When the Ring came out of the fire at Bag End there were
lines of fire that seemed to form the letters of a flowing script. They shone piercingly bright,
The elves crossing the Shire
bore no lights, yet as they walked a shimmer, like the light of the moon above the rim of the hills before it rises, seemed to fall about their feet.
In the barrow, Frodo
thought there were two eyes, very cold though lit with a pale light that seemed to come from some remote distance.
and
for the light seemed to be coming out of himself, and from the floor beside him, and had not yet reached the roof or wall. He turned, and there in the cold glow he saw lying beside him Sam, Pippin, and Merry.
At the Ford of Bruinen
there came a plumed cavalry of waves. White flames seemed to Frodo to flicker on their crest
At the doors to Kazadh-dum
Then slowly on the surface, where the wizard's hands had passed, faint lines appeared, like slender veins of silver running in the stone. At first they were no more than pale gossamer-threads, so fine that they only twinkled fitfully where the Moon caught them, but steadily they grew broader and clearer,
and later, still in Kazadh-dum
In his left hand he held up his glimmering staff, the light of which just showed the ground before his feet;
Crossing the Dead Marshes
other [lights] appeared soon after: some like dimly shining smoke, some like misty flames flickering slowly above unseen candles
In Fangorn forest, Fangorn
held his hands over them, and immediately they began to glow, one with a golden and the other with a rich green light; and the blending of the two lights lit the bay; as if the sun of summer was shining through a roof of young leaves. Looking back, the hobbits saw that the trees in the court had also begun to glow, faintly at first, but steadily quickening, until every leaf was edged with light: some green, some gold, some red as copper; while the tree-trunks looked like pillars moulded out of luminous stone.
When Aragorn, et all, met Gandalf the White at the edge of Fangorn
The sword of Aragorn, stiff in his motionless hand, blazed with a sudden fire.
The Palantir fell at Orthanc and
the ball was unharmed: it rolled on down the steps, a globe of crystal, dark, but glowing with a heart of fire.
and a bit later
At first the globe was dark, black as jet, with the moonlight gleaming on its surface. Then there came a faint glow and stir in the heart of it
In Morgul Vale
Down in its depths glimmered like a glow-worm thread the wraith-road from the dead city to the Nameless Pass. He turned hastily away.
When Denethor unveiled the Palantir
as he held it up, it seemed to those that looked on that the globe began to glow with an inner flame,
At Battle of the Pelinor Fields, Merry
drew his sword and looked at it, and the intertwining shapes of red and gold; and the flowing characters of Númenor glinted like fire upon the blade.
In front of the Black Gate, Gandalf
cast aside his cloak and a white light shone forth like a sword in that black place.
Then, there was Gollum whose
two pale lamplike eyes shone coldly as they peered inside,
(They did this a lot. Must have made hunting harder.)
(And I won't even try to list all the examples of Phial of Galadriel shining or swords which shone blue near orcs.)
The point of this long (but incomplete) list is that, while some of these examples can be discounted as literary, many cannot as they explicitly speak of the conjured lights as illuminating their environment.
This establishes beyond doubt that (a) magic/the supernatural is associated with uncanny light effects and (b) the effects are many and varied and in-the-story-real.
The fire on the Head Nazgul's sword can't be proven to be in-the-story-real, but, equally, can't be proven not to be. (FWIW, my own bet is that it was "real".)
