Attaching CSS to block doesn't work mymodule is in modules/custom directory.

$block['content']['#attached']['css'][] = drupal_get_path('module',
                                          'mymodule' . '/mymodule.css');


mymodule.css exists.

However file is not attached.
You have a typo in your statement. It should be:

$block['content']['#attached']['css'][] = drupal_get_path('module', 'mymodule') . '/mymodule.css';


Also, be sure to clear the cache after you add statements like this to your code.