Hi, I was recently able to add "teal" divider lines in my sidebar (& above footer area) using HTML WIDGETS at a wp.com blog using Triton Lite Theme, see example here on a post page (as the sidebar is not visible on home page):
I found the html coding by googling for "wordpress.com - divider lines" & used suggestions from a site called wpbtips.wordpress.com, though there are other tip sites as well. Sample google search:
You have to change the width in the code to jive with the width of your theme's sidebar. You can also change the height of the line (thickness). You can also change the color of the line with whatever HEX color code.
I was thrilled to learn this trick using html widgets for wp.com freebie/no-css-upgrade blogs.
With that being said, I tried the same code (adjusting width, etc.) in a different theme (P2) & it didn't work. No line appeared. :-/ So maybe it only works on some themes & not others(?) Or maybe I didn't do it correctly for the P2 theme/haven't had time to go back & figure it out.
I would post the code here but from what I understand, posting actual code in forums can mess up the forum(???)
Also, you said you wanted a "blank line," so not sure we are talking about the same thing. I wanted clearly visible colored line dividers. :)