Add ‘/blog/’ Prefix in Blog Posts

register_post_type.php

<?php
$labels = [
    "name" => __('Services', 'text-domain'),
    //...
    //...
];

$args = [
    "label" => __('Services', 'text-domain'),
    "labels" => $labels,
    //...
    //...
    "rewrite" => ["slug" => "service", "with_front" => FALSE], //<-- SET with_front to FALSE
    //...
];
register_post_type("service", $args);