Use Config in Block Display

To make use of the configuration of instances of the block, we can modify the build() method of the HelloBlock class:

  /**
   * {@inheritdoc}
   */  
  public function build() {
    $config = $this->getConfiguration();
    
    $name = $config['hello_block_name'] ?? $this->t('to no one');

    return [
      '#markup' => $this->t('Hello @name!', [
        '@name' => $name,
      ]),
    ];
  }