How can I extend custom class extending BlockBase in contrib module?

The block opigno_documents_last_group_block is located in opigno_learning_path/src/Plugin/Block/DocumentsLastGroupBlock.php, which looks like this:

<?php

namespace Drupal\opigno_learning_path\Plugin\Block;

use Drupal\Core\Block\BlockBase;
use Drupal\Core\Url;

/**
 * Provides a documentslastgroupblock block.
 *
 * @Block(
 *   id = "opigno_documents_last_group_block",
 *   admin_label = @Translation("DocumentsLastGroupBlock"),
 *   category = @Translation("Custom")
 * )
 */
class DocumentsLastGroupBlock extends BlockBase {

  /**
   * @var \Drupal\Component\Plugin\Context\ContextInterface[]|mixed
   */
  protected $groupId;

  /**
   * {@inheritdoc}
   */
  public function build() {
    $this->groupId = $gid = $this->configuration["group"];
    $tid = _tft_get_group_tid($gid);
    $content = _tft_folder_content($tid, FALSE, $gid);
    $content = array_slice($content ?: [], 0, 4);
    foreach ($content as $index => $item) {
      $content[$index] = [
        '#theme' => 'opigno_documents_last_group_item',
        '#type' => $item["type"] == 'file' ? 'file' : 'folder',
        '#item' => $item,
        '#label' => $item["name"],
        '#link' => $this->itemLink($item),
      ];
    }
    $build['content'] = [
      '#theme' => 'opigno_documents_last_group_block',
      'content' => $content,
    ];
    return $build;
  }
(...)

and is attached in LearningPathController in this module:

 /**
  * Training document block.
  */
  public function trainingContentDocuments(&$content, $group) {

    // $TFTController = new TFTController();
    // $listGroup = $TFTController->listGroup($group->id()); 
    $tft_url = Url::fromRoute('tft.group', ['group' =>    $group->id()])->toString();

    $content['tabs'][] = $tft_url = [
     '#markup' => '<div class="see-all see-all-files"><a href="' . $tft_url . '">' . $this->t('See all') . '</a></div>',
    ];

    $block_render =    $this->attachBlock('opigno_documents_last_group_block', ['group' => $group->id()]);
   $block_render["content"]['link'] = $tft_url;
    $content['tab_content']['documents'] = (isset($block_render["content"]["content"]) && !empty($block_render["content"]["content"])) ? [
      '#type' => 'container',
      '#attributes' => [
        'id' => 'documents',
      ],
      'block' => [
        'content' => $block_render["content"],
      ],
    ] : [];

  }

I tried to extend it in custom module: web/modules/custom/npx_files/src/Plugin/Block/NpxFilesDocumentsLastGroupBlock.php:

<?php

namespace Drupal\npx_files\Plugin\Block;

use Drupal\opigno_learning_path\Plugin\Block;

class NpxFilesDocumentsLastGroupBlock extends DocumentsLastGroupBlock {

  public function build(){
    return [
      '#markup' => $this->t('Hello, World!'),
    ];
  }

}

However, it doesn't trigger the block, what is the good way to extend this? Cache cleared&module turned on of course.