Can't change sidebar on main blog posts page

Very strange problem this one. I have got the plugin working on all pages and posts apart from the page I make the posts page in Reading Settings.

I have a page called Blog which works fine selecting a custom sidebar as it's using the page.php template. When i make it the posts page it will then use the index.php template which then just shows the default navbar. I can't do anything to change this.

I have tried debugging using get_post_custom($post_id) to show the custom fields on the page and noticed there is an element for custom sidebars shown

[_cs_replacements] => Array
[0] => a:1:{s:9:"sidebar-1";s:4:"cs-1";}

On any post/page this will update to cs-2, cs-3 if i change the sidebar and if i make the sidebar the default one, the _cs_replacements element is removed from the fields array

On the posts page, however, this doesn't update at all but also it is ignored when i change the sidebar on the page editor. Again, if i revert to the page just being a page, it works fine.

Any ideas?