Hey @damien,
Okay, try these three steps for the previews:
1. Configure your link-resolver
In your project files, it doesn't look like your link resolver (~/plugins/link-resolver.js) is configured for your blog posts. You'll want to add something like this:
if (doc.type === 'blog_post') {
return '/blog/' + doc.uid
}
2. Make sure previews are configured correctly in Prismic
In Dashboard > Settings > Previews, make sure that all of your previews are properly configured, with /preview added for the Link Resolver.
3. Delete your preview cookie
On your website (localhost or production), open Dev Tools, go to the "Application" tab, and click "Cookies" in the side menu. Click on the domain (localhost:8000 or your production domain), and scroll to find any domains that include io.prismic.preview. Delete all of those.
Finally, refresh everything and try opening a preview from the writing room again.
Let me know if that helps. If not, I can do some more digging to see if I can find anything else that might help. And, in any case, let me know if you have any other questions.
Best,
Sam