Thanks Sam - that helped move us a little further.
We populated the "author" on that blog post, and that has published fine now. Also the preview (for an unpublished post) now works if navigating to it, but when previewing directly it still takes us to domain.com/post-title instead of domain.com/blog/post-title
(and manually editing the URL doesn't work).
So, we can successfully publish content, and have a way to preview - but something is still not working 100% in terms of the preview. How do we get Prismic to take us to the right place / do we have something messed up in terms of routes on the Nuxt end for preview to work smoothly?