How to get a custom block form in another custom block?
The answer is to use the plugin.manager.block service
Example:
In the Block B BlockForm method.
public function blockForm($form, FormStateInterface $form_state) {
  $block_manager = \Drupal::service('plugin.manager.block');
  $block_a = $block_manager-&gt;createInstance('block_a');
  $form = block_a-&gt;blockForm($form, $form_state)
  return $form;
}