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 $form The renderable form array representing the entire configuration form.
Overrides BlockBase::blockForm
\Drupal\block\BlockBase::form()
public function blockForm($form, &$form_state) {
  $form['display_message'] = array(
    '#type' => 'textfield',
    '#title' => t('Display message'),
    '#default_value' => $this->configuration['display_message'],
  );
  return $form;
}