Force block page to use admin theme? I'm trying to use this snippet I found, but it throws this error:

function mytheme_init() {
  // Hardcode the administration theme onto the blocks page
  if ($_GET['q'] == 'admin/build/block') {
    global $theme;
    $theme = 'rubik';
  }
}


Fatal error: Unsupported operand types in /var/www/clients/client4/web39/web/modules/block/block.admin.inc on line 39

The $theme = 'rubik'; line appears to be the culprit in some way, as it doesn't throw the error if I comment it.
You have to use the global variable $custom_theme.
The implementation of your hook_init() is good.