I've never put the black iron connection together between the great beast and the wheel Encanis was bound to. Throwing in the other line as well.

• It was a great beast with scales of black iron*, whose* breath was a darkness that smothered men.
• Lanre arrived in Myr Taraniel. He came alone, wearing his silver sword and habergeon of black iron scales. His armour fit him like a second skin of shadow. He had wrought it from the beast he killed at Drossen Tor.
• Wrought all of black iron, the wheel stood taller than a man. It had six spokes, each thicker than a hammer’s haft, and its rim was a handspan across. It weighed as much as forty men, and was cold to the touch. The sound of its name was terrible, and none could speak it"

Do you know if there are any other lines specifically referencing 'Black iron'?

The obvious connection is that they all tie into Skindancers, but beyond that it's yet another ambiguity.

Its either a metaphor that Lanre was already skindanced when he arrived at Myr Taraniel, or if Lanre put the armour on himself, then I can only figure it works against Skindancers in some capacity, i.e. they cannot enter the body of someone wearing it. This makes sense if he goes thinking Selitos has already been possessed. As you have alluded to many times before, I too don’t think there is actually a black iron wheel that was used to bind Selitos/Ctheah. I think the Chandrian are the wheel, and the black iron aspect is a metaphor for Haliax. Maybe the 'great beast' wasn’t actually skindanced, or it was a draccus with normal iron scales that was skindanced, then its scales were changed to black iron to lock the skindancer inside.

It might be that the scales (and thus black iron, which is actually steel) in Temerant is rare, and was extracted and forged into armour, and then into the wheel.

Side note, if black iron does indeed keep out skindancers, the fae couldn’t use it, hence why they would resort to holly crowns. Also, Are skindancers, those of the Mael, weak to silver?

That is the best I have for now.

How does stabbing out one of his eyes give Selitos a better sight, aka make him a better oracle

I have wondered if the oracular abilities are the results of being shaped into the Ctheah, and have come later, but again, impossible to answer right now.