drupal8_theme_hook.php

<?php
/**
 *  @author 
 * https://drupal.stackexchange.com/questions/192616/how-to-make-a-theme-hook-suggestion-for-blocks-according-to-region/192623#192623
 */


use Drupal\block\Entity\Block;

function MODULE_theme_suggestions_block_alter(array &$suggestions, array $variables) {
  if (!empty($variables['elements']['#id'])) {
    $block = Block::load($variables['elements']['#id']);
    $suggestions[] = 'block__' . $block->getRegion() . '__' . $variables['elements']['#id'];
  }
  /* Use this 'else if' only if you are using page_manager module and want to know which region is the block */
  else if (isset($variables['elements']['#configuration']['region'])) {
    $suggestions[] = 'block__page_' . $variables['elements']['#configuration']['region'] . '__' . end(explode(':', $variables['elements']['#plugin_id']));
  }
  return $suggestions;
}