How do I pre-configure default blocks for my drupal 7 theme
You can't control this from the theme at the time it's being enabled.

Drupal will try to assign each block enabled for the default theme to the same region in your theme, if available. If there's no region with a coincident name, it will drop the block to the default region, that is the first region defined in your theme's .info.

So there're two things you can do:


use more standard region names (sidebar_first, sidebar_second, footer)
in theme's .info, declare in the first place the more suitable region for orphaned blocks