Block templates by bundle name

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 * Courtesy @nicrodgers
 */
function MYTHEME_theme_suggestions_block_alter(array &$suggestions, array $variables) {
  if (isset($variables['elements']['content']['#block_content'])) {
    $block_content = $variables['elements']['content']['#block_content'];
    $suggestions[] = 'block__' . $block_content->bundle();
  }