The answer is unclear and seems largely up to interpretation
In an older version of this part of the chapter, the imagery of the flames between the crown and the shoulders of the Witch King and those of his sword are omitted:
“The Black Rider [?lay for laid] back his hood and ..... crown that sat upon no visible head save only for the light of his pale eyes.(19) A deadly laughter [?rang] out.
'Old fool,' he said. 'Old fool. Do you not know death when you see it? Die now and curse in vain. This is my hour of victory.' And with that he lifted his great sword. [Added: And then suddenly his hand wavered and fell and it seemed that he shrank.]
The History of Middle-earth, Volume VII: The War of the Ring, Part Three: Minas Tirith, VI The Siege of Gondor
While this passage seems to be reasonably different to that in the Lord of the Rings (LR), it was the most recent form the passage took before the version seen in LR.
From short passages of further drafting, either separate or pencilled on the fair copy manuscript itself and then overwritten, the final form of the story was largely reached, and there is nothing to notice in this development. But as the fair copy was left to stand there remained a differences from RK.
The only note on the topic is note 19, which seems to discuss only points from before the mention of the flames.
The handwriting here is such that many words could not be interpreted at all in isolation, without context or other clues, but 'save only for the light of his pale eyes' seems tolerably clear. Cf. p. 365.
It seems to be up to the interpretation of the reader. The many examples of LR depicted in visual media seemed to have adapted the "flaming sword" stance however it is entirely plausible that this is purely down to reflections of the fire. There is however in the final version and the earlier version specific mention made to the bursting of the gate asunder, and the fires through which the "Black Horseman" came. As such it seems increasingly likely that it is merely reflections and not a literal flaming sword.
There Gandalf stood. And then over the hill in the flare of the fire a great Black Horseman came. [...] Thrice the rams boomed. Thrice he cried, and then suddenly the gate as if stricken by some blast burst [?asunder], and a great flash as of lightning, burst and fell, and in rode the Lord of the Nazgul.