Returns the configuration form elements specific to this block plugin.
Blocks that need to add form elements to the normal block configuration form should implement this method.
array $form: The form definition array for the block configuration form.
array $form_state: An array containing the current state of the configuration form.
array $form The renderable form array representing the entire configuration form.
\Drupal\block\BlockBase::form()
public function blockForm($form, &$form_state) {
return array();
}