block_permissions.module

<?php
/**
* @file
* This module provides a block view permission and an option to override this
* permission for each block.
*/

use \Drupal\Core\Access\AccessResult;
use \Drupal\Core\Session\AccountInterface;
use \Drupal\block\Entity\Block;

/**
 * Implements hook_block_access().
 */
function block_permissions_block_access(Block $block, $operation, AccountInterface $account) {
	if ($operation == 'view') {
		$settings = $block->get('settings');
		if ($settings['override_block_visibility_permission']) {
			return AccessResult::allowed();
		}
		else if (!$account->hasPermission('view blocks')) {
			return AccessResult::forbidden();
		}

	}

	// No opinion.
	return AccessResult::neutral();
}

/**
 * Implements hook_form_alter().
 */
function block_permissions_form_block_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
	$block_editing = Block::load($form['id']['#default_value']);
	$settings = $block_editing->get('settings');

	// Add a config to override the permission for all blocks
	$form['settings']['override_block_visibility_permission'] = array(
		'#type' => 'checkbox',
		'#title' => t('Yes'),
		'#prefix' => '<div class="override-block-view-permissions-description">' . t('Override the block view permissions ?') . '</div>',
		'#default_value' => isset($settings['override_block_visibility_permission']) ? $settings['override_block_visibility_permission'] : 0,
	);

	// Add a custom submit handler to save the array of types back to the config file.
	$form['actions']['submit']['#submit'][] = 'block_permissions_block_settings_submit';
}

/**
 * Custom submit handler for hook_form_alter().
 */
function block_permissions_block_settings_submit($form, FormStateInterface $form_state) {
	$block = Block::load($form['id']['#default_value']);
	$settings = $form_state->getValue('settings');

	$block->set('settings', $settings);
	$block->save();
}