Mostly this is info from the UG.
CodeIgniter can be told to load a default controller when a URI is not present, as will be the case when only your site root URL is requested. To specify a default controller, open your
application/config/routes.php file and set this variable:
$route['default_controller'] = 'Blog';
Blog is the name of the controller class you want used. If you now load your main index.php file without specifying any URI segments you’ll see your Hello World message by default.
I guess it doesn’t matter if it’s blog or Blog. I’ve seen it work both ways in this UG.