Theme suggestion for D8 blocks

/**
* Implements hook_theme_suggestions_HOOK_alter() for form templates.
* @param array $suggestions
* @param array $variables
*/
function THEMENAME_theme_suggestions_block_alter(array &$suggestions, array $variables) {
// Block suggestions for custom block bundles.
if (isset($variables['elements']['content']['#block_content'])) {
array_splice($suggestions, 1, 0, 'block__bundle__' . $variables['elements']['content']['#block_content']->bundle());
}
}