node_preprocess_block

node_preprocess_block(&$variables)

function node_preprocess_block(&$variables) {
  if ($variables['configuration']['provider'] == 'node') {
    switch ($variables['elements']['#plugin_id']) {
      case 'node_syndicate_block':
        $variables['attributes']['role'] = 'complementary';
        break;
    }
  }
}