Create TWIG template suggestions for Drupal 8 custom block types. Enables block--custom-block-type.html.twig

HOOK_theme_suggestions_block_alter

function basic_theme_suggestions_block_alter(array &$suggestions, array $variables) {

  $block = $variables['elements'];
  $blockType = $block['#configuration']['provider'];
  
  if ($blockType == "block_content") {
    $bundle = $block['content']['#block_content']->bundle();
    $suggestions[] = 'block__' . $bundle;
  }

}