EntityBlock.php

<?php

namespace Drupal\block\Plugin\migrate\destination;

use Drupal\migrate\Plugin\migrate\destination\EntityConfigBase;
use Drupal\migrate\Row;

/**
 * @MigrateDestination(
 *   id = "entity:block"
 * )
 */
class EntityBlock extends EntityConfigBase {
    
    /**
     * {@inheritdoc}
     */
    protected function getEntityId(Row $row) {
        // Try to find the block by its plugin ID and theme.
        $properties = [
            'plugin' => $row->getDestinationProperty('plugin'),
            'theme' => $row->getDestinationProperty('theme'),
        ];
        $blocks = array_keys($this->storage
            ->loadByProperties($properties));
        return reset($blocks);
    }

}