Blocks page and Themekey module
No, there is no way you can change theme on this location using themekey. 

You might consider to do this in your custom module. Something like this:

function YOUR_MODULE_NAME_init()
{
  global $custom_theme;
  if($_GET['q'] == 'admin/build/blocks'){
    $custom_theme = 'your_theme_name';
  }
}