I am working on a accordion_blocks module. For this module I wanted to enable the accordion effect for the blocks appearing in left and right regions. To do that I need to find our the regions of the theme. To find out the regions I used bellow code in the hook_theme_registry_alter function.
// Populate all block regions.
$regions = system_region_list($theme);
In the template_preprocess_page hook, I directly used $variables[‘left’], $variables[‘right’]