Translation of BlockBase (ConfigurablePluginInterface)
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