I got that, and that obsidian is quite beautiful, but in this case it still leaves it ambiguous with respect to magic. Some lost craft, even if from Numenor near its height, still leaves the possibility it's just some wondrous lost skill which only seems like magic, like the roman recipe for water resistant concrete. It's a distinctive monument like a henge. If one were to push that idea (imo too far), one might even be tempted to apply to Feanor in Aman, though I think the firstborn are inherently magical with mystical powers unattainable by men.

The Barrow blades could be similar, merely excellent craftsmanship combined with reverence. The spells of Arnor blades might only be incantations and runes, only requiring faith and courage from a wielder, with maybe a good helping of providence. Did the Witchking have an Achilles heel? Although that doesn't explain Sting can glow in the dark when enemies are about, nor explain how a particularly good grade of steel can slice ghostly sinew, albeit there are real world superstitions and folklore about Iron, claiming it had the power to ward off evil things like witches and ghosts. Maybe some of it is ascribable to an origin with or proximity to the elves in the second age (some of their craft magic rubbed off?), and is just another proof of their waning influence on middle earth, magic waning along with their departure.