Personally I do not like giving block access to website editors. In this one case, there was a need to edit custom blocks. I granted access to administer blocks, but I blocked access to the overview page with this EventSubscriber.

First, add a file called mymodule.services.yml. Add the following:

services:
  mymodule.event_subscriber:
    class: Drupal\mymodule\EventSubscriber\CustomRedirects
    tags:
      - {name: event_subscriber}

Then inside mymodule\src\EventSubscriber\CustomRedirects.php write:

<?php

namespace Drupal\mymodule\EventSubscriber;

use Drupal\Core\Url;
use Symfony\Component\HttpFoundation\RedirectResponse;
use Symfony\Component\HttpKernel\KernelEvents;
use Symfony\Component\HttpKernel\Event\GetResponseEvent;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
 * Class CustomRedirects
 * @package Drupal\mymodule\EventSubscriber
 *
 * Redirects editors to custom blocks on the block overview page
 */
class CustomRedirects implements EventSubscriberInterface {

  public function checkForRedirection(GetResponseEvent $event) {

    $request = $event->getRequest();
    $path = $request->getRequestUri();
    if((strpos($path, 'structure/block') !== false) && (strpos($path, '/block-content') === false)) {
      $roles = \Drupal::currentUser()->getRoles();
      if(in_array('editor', $roles)) {
        // Do something if user has role editor
        $newPath = '/admin/structure/block/block-content';
        $new_response = new RedirectResponse($newPath, '301');
        $new_response->send();
      }

    }

  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    //The dynamic cache subscribes an event with priority 27. If you want that your code runs before that you have to use a priority >27:
    $events[KernelEvents::REQUEST][] = array('checkForRedirection', 29);
    return $events;
  }

}

A sidenote here: this isn't really preventing editors from editing blocks. If they would have a directlink to /block/edit/whatever-core-block they have access. It is rather a utility help to prevent editors from messing up blocks.