Translation of BlockBase (ConfigurablePluginInterface) public function blockForm($form, FormStateInterface $form_state) {
    $form = parent::blockForm($form, $form_state);

    // Retrieve existing configuration for this block.
    $config = $this-&gt;getConfiguration();

    // Add a form field to the existing block configuration form.
    $form['Heading'] = array(
      '#type' =&gt; 'textfield',
      '#title' =&gt; t('Heading'),
      '#default_value' =&gt; isset($config['heading']) ? $config['heading'] : '',
    );


'#default_value' => isset($config['heading']) ? $config['heading'] : '',

can this thing be translated? And how?
I use it to display a text to site users
The key was in creating mymodule.schema.yml in mymodule/config/schema/mymodule.schema.yml

with content of a kind 

block.settings.myblock:
  type: block_settings
  mapping:
    Heading:
      type: text
      label: 'Heading'
      translatable: true


also 

$config = $this-&gt;getConfiguration();


is not needed
using 

$this-&gt;configuration['heading']


insted

\core\modules\system\src\Plugin\Block\SystemBrandingBlock.php -- is a good example