I have a template set for blog posts, unfortunately, it doesn’t work on the blog page I chose from the WordPress settings but it works for category/tag pages.
Post images displayed on archive pages don’t have a way to shape the border-radius and other border kinds of stuff.
Please have a look at this article in the academy about how to set up the blog page.
If you can provide me with some more information about the concrete issue(s) you are having regarding borders on archive pages I’m happy to help. Ideally create a small screencast and explain what you’re trying to do and what does not work.
I am not able to see what you did by looking at a frontend screenshot. Please record a small screencast or show and explain your exact setup within Bricks.