how can I add a dotted line separating blog posts in all posts view

Right now, there is no kind of divider between blog posts in the full post blog view. Is it possible to add a dotted line separator between stacked posts? 

There is if you use a blog summary page as opposed to the blog listing page. You have a lot more flexibility for a lot of changes using one of those. You put the actual blog page in the unlinked section of your site, which takes it off the nav bar,  and use the summary page as the blog page instead. You can then add solid lines dotted lines and other things. 

Can you share link to blog page? We can help easier