I was trying to apply custom master pages to the blog sites.
Copied the custom master page which I used for the main site over to the blog site via SPD. Right clicked on it to set as both Default Master Page and Custom Master Page.
Worked on the blog’s default page but also effected the system admin page.
From this post, it seems that it doesn’t have a fix for it.
Other Blog Customization articles found:
How to customise SharePoint 2010 Blog Post
How to Customize Blog Posts in Sharepoint 2010
Customize SharePoint 2010 Blog Post