(Drupal 8.x) Provide template suggestions for custom block types.

mytheme.theme

<?php

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 *
 * Original source: @jeff-burnz https://www.drupal.org/node/2724333#comment-11184655
 */
function mytheme_theme_suggestions_block_alter(array &$suggestions, array $vars) {

  // Template suggestions for custom block bundles.
  if (isset($vars['elements']['content']['#block_content'])) {
    array_splice($suggestions, 1, 0, 'block__bundle__' . $vars['elements']['content']['#block_content']->bundle());
  }
}