New theme - getting 400 error 'blog.html.twig' is undefined

I've read the previous threads that seemed relevant - tried a few changes but none did anything.

So I switch from Quark to Masonry and get this error. It's not a particularly helpful error. The only thing I can see is that the Masonry theme does not have a blog.html.twig template. It does, however, have a post.html.twig template. Should I rename it? Should I change something in my post files? Should I rename my post files?

Really lost here...