Extending block configuration schema
It would really help to knowing how your data is saved (e.g. providing a snippet from your BlockBase::blockSubmit() implementation that shows this value), or what exactly you tried adding to your yml files.

However, in theory a working example that assumes, you are saving the value like this:

PmInboxBlock.php

/**
 * @Block(
 *   id = "pminbox",
 *   admin_label = @Translation("PmInbox"),
 *   category = @Translation("PmInbox"),
 * )
 */
class PmInboxBlock extends BlockBase {

  // ... Your Form definition here ...

  /**
   * {@inheritdoc}
   */
  public function blockSubmit($form, FormStateInterface $form_state) {
    $this-&gt;configuration['thread_count'] = $form_state-&gt;getValue('thread_count');
  }

}


Then you provide the schema definition in a schema file named after your module:

mymodule.schema.yml

block.settings.pminbox:
  type: block_settings
  label: 'PmInbox Block'
  mapping:
    thread_count:
      type: string
      label: 'Thread count'