Plugin Development Output

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It appears you are missing your plugin folder when enqueueing your scripts.
plugins_url outputs the URL to your wp plugins folder it does not include your custom plugin folder. 

wp_register_script( 'script_js', plugins_url('/YOUR_FOLDER/js/script.js')); // Add your plugin folder
wp_register_script( 'script_js', plugins_url('/js/script.js',__FILE__)); // Let php add your folder

Also you don’t need to register your scripts / styles first you can just enqueue them straight up. Registering them first is only useful if they are dependencies of other scripts.

Eg.

add_action('wp_enqueue_scripts', 'enqueue_style');
function enqueue_style(){
  wp_enqueue_script( 'script_js', plugins_url('/js/script.js', __FILE__ ) );
  wp_enqueue_style( 'style_css', plugins_url('/css/style1.css', __FILE__ ) );
}

And now for your shortcode output you need to capture the output with the php function ob_start()

function onScrollBlur(){
  ob_start();
  ?>
  <div class="blurImg">
    <div style="background-image: url('https://d262ilb51hltx0.cloudfront.net/fit/c/1600/1280/gradv/29/81/60/darken/25/1*4ncz3hLxmL8E_bUh-0z62w.jpeg')"></div>
    <div class="blur" style="background-image: url('https://d262ilb51hltx0.cloudfront.net/fit/c/1600/1280/gradv/29/81/40/darken/50/blur/50/1*4ncz3hLxmL8E_bUh-0z62w.jpeg')"></div>
  </div>
  <header>
    <div>
      <h1>
        Medium
      </h1>
      <p>
        Everyone’s stories and ideas
      </p>
      <a href="https://medium.com/" title="Medium">Learn more</a>
    </div>
  <?php
  return ob_get_clean();
}

You can read more about the Shortcode API here: https://codex.wordpress.org/Shortcode_API#Output