site/_includes/blog_contents.html

<div>
  <aside>
    <h4>Posts</h4>
    <ul>
        {% for post in site.posts %}
        <li class="{% if page.title == post.title %}current{% endif %}">
          <a href="{{ site.baseurl }}{{ post.url }}">{{ post.title }}</a>
        </li>
        {% endfor %}
    </ul>
  </aside>
</div>