>> I wonder, though, if part of the point of the original post, was that white is seen as normal and not needing description and non-white as departing from the norm and needing to be described so that the reader knows in just what way it departs from the norm and how far. <<

It is.

>> I wonder what it would be like to read a story in which the default was black, and only the (handful of) white characters came in for elaboration about their skin tones. <<

Oh, I've done that. It's one of the things that makes people think that I and my writing are weird. Only in recent times have I started seeing a stream of books that are actually set in places where all the people are brown or black and it's NOT about race. Nigerian SF, for instance.