The Blog area of the option page is dedicated to manage options for individual posts pages and blog archives.
Single post tab, you’ll find options that allow you to switch on or off the display of some elements regarding the post when it’s displayed in its individual page, reachable from its permalink (they have no effect on loops).
Each option you see listed in this tab, can be overridden by each post in its editing screen in the admin area.
You’ll also be able to specify if all posts have, by default, a sidebar associated to them. This option is common to all blog posts, but, again, it can be overridden in the post editing screen in the admin area.
Also concerning sidebars, there’s a specific part of this document that’s dedicated to explaining how they work.
Archives tab, you’ll find options that allow the control of the loop in archive pages (date archives, categories and tags are treated as one), as well as to which widget area, if any, to show in those pages.