As I have understood, you wants to go to administration/blog.aspx page while entering into admin area. In this case you should change a URL for "Administration" hyperlink on front end. Go to Modules/Header.ascx, find a "Page.ResolveUrl("~/administration/")" and change it to "Page.ResolveUrl("~/administration/blog.aspx")". Now when you will click the "Administration" link you will be redirected to "administration/blog.aspx" page.