"Block a website" script with vanilla JavaScript
The main suggestion I have is to take the values from sitesToBlock and put them in the @match metadata key instead of detecting when to update the document content. There is an extensive guide about match patterns here. With this approach there should not be any need to wait for the DOM to be loaded and check if the URL contains elements in sitesToBlock, though maybe that “news_website_1” loads slower than the one I tested with and that won’t work.

Other review points:


default to using const instead of let when declaring variables unless re-assignment is mandatory. This helps avoid accidental re-assignment.
use for...of loops when possible- e.g. for iterating through sitesToBlock and there is no need for a counter variable (e.g. i) other than accessing the current element
&lt;script&gt; tags still executed - those tags outside the body tag (e.g. in the HTML &lt;head&gt; element, or anywhere else) are still executed so it might make sense to remove those, otherwise the execution of a script could modify the document body after the user script has finished updating the document body, which likely would be undesirable. 


Below is the script as I would update it. I know the domain might include www or another sub-domain if the site redirects to one - I modified the regex to look specifically for a final dot followed by anything that isn’t a dot and an end of string anchor (i.e. $). 

// ==UserScript==
// @name         page blocker
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  block pages
// @author       Sᴀᴍ Onᴇᴌᴀ
// @match        *://*.news_website_1.com/*
// @match        *://*.news_website_2.com/*
// @grant        none
// ==/UserScript==

(() =&gt; { //IIFE arrow function
    'use strict';

    const regexToMatchTLD = /\.[^.]+$/;
    const domain = location.hostname.replace(regexToMatchTLD, '');; 
    document.body.innerHTML =`
          &lt;div style="direction: ltr; position: fixed; top: 0; z-index: 999999; display: block; width: 100%; height: 100%; background: red"&gt;
            &lt;p style="position: relative; top: 40%; display: block; font-size: 66px; font-weight: bold; color: #fff; margin: 0 auto; text-align: center"&gt;
              The website ${domain} successfully blocked !
            &lt;/p&gt;
          &lt;/div&gt;
    `;
})();