There’s no need to clone the “blank” theme, because Micro.blog automatically applies any of the templates in “blank” to your theme if they don’t exist. It does this every time you post to your blog. You should only include those templates in your theme if you really need to modify their behavior, like a very special JSON Feed or archive page.
As for getting started… I think I would start with the “default” theme and customize from there. Most of the themes are based on other Hugo themes and customized for Micro.blog. So if there’s a similar Hugo theme you can find (on https://themes.gohugo.io) that is another way to start.
We definitely need more guides for themes so let us know if you have any questions!