Default theme region can be hidden

- Install a fresh Drupal 7.
- Add a custom theme, such as one generated by drupalgardens.com
- See how many of your blocks appear on the top of the page (expected that the first block region is choosen for blocks in regions not present in the now enabled theme)
- Go to blocks admin and see how there is no option whatsoever to reposition these blocks

The reason this happens is that the theme defines regions from top to bottom and starts off with 'page_top'. Since Drupal itself defines 'page_top' as a hidden region, it will not show up on the blocks admin page. Therefore blocks assigned to this region are not capable of being administered. This is all fine though. The issue is that blocks should not be put there by Drupal itself, and the root cause is that Drupal will select the first defined block region, despite it possibly being disabled. The solution is very simple, attached.