Meta tags on Drupal blog home page
You need to use the metatag_context sub module of Metatag, which means you need the Context module. 

I was thrown for a loop on this as well. It'll create a default Context for a few of these oddball situations. Your metatag information is then controlled through Context.

See source for export handling blog: http://cgit.drupalcode.org/metatag/tree/metatag_context/metatag_context.context.inc#n251