In BlogML page , they have BlogML for CS2.1 and CS2007. But they can't works in CS2008 ( or CS2008.5 ).
I modify some code for cs2008.5 and wrote a new import and export page.
After install it , you can go to /ControlPanel/BlogAdmin/Blogs.aspx and find BlogML Import/Export button.
Download page : Codeplex - BlogML Release page