Add a rule like this to the Main Options : Post Specifics : Post Area BG CSS+:
{border: 1px solid #000000;padding: 8px 1% 8px 1%;} .single %selector% {border:none;padding:0;}
This is assuming that you have your blog views (default blog or Page with Posts) set to show excerpts, and will see the full view only on the single page.
With this CSS+, you would not check the border option for posts since the rule does it. When you add a border to the blog post view, you want some padding. The second part of that rule handles the single view, and removes the border and the padding. You can adjust these as you see fit, including the border width, type, and color.