Template suggestions for Drupal 8 custom block types

function THEMENAME_theme_suggestions_block_alter(array &$suggestions, array $variables) {
  if (isset($variables['elements']['content']['#block_content'])) {
    $suggestions[] = 'block__' . $variables['elements']['content']['#block_content']->bundle();
  }
}

just a quick tweak so that you can still override a specific instance of a custom block:

function THEMENAME_theme_suggestions_block_alter(array &$suggestions, array $variables) {
  if (isset($variables['elements']['content']['#block_content'])) {
    array_splice($suggestions, 1, 0, 'block__' . $variables['elements']['content']['#block_content']->bundle());
  }
}

Thanks, works great!

Thanks @markconroy, that was exactly what I needed!