It might be literal and descriptive, at least in-world. The spires are made of a translucent black stone that lets light through. I imagine it as looking like black obsidian in that way, which is also called Volcanic Glass. In most of the book it's called Spirestone, but in the first four chapters it's referred to as Cinderstone. So Cinder Spires could literally be Spires of Cinderstone. It's heavily implied that Cinderstone is not a natural material we are aware of, much like the etheric crystals are not, but it may be a colloquial word similar to Volcanic Glass.
Of course, that would be an in-world explanation. There is no denying that it is an evocative choice of phrasing that would fit well to the implied End of Days scenario that is coming.
