CleanSlate (Liquid): Simplified example of how to pull a featured blog post.

_featured-blog-post-simple-liquid.html

{% assign blog = site | get_page: 123 %} <!-- Change this ID to your blog_index page's ID -->
{% assign articles = blog.articles | filter_articles: tags: "featured", limit: 1 %}
{% for article in articles.all %}
  <h1>{{ article.name }}</h1>
{% endfor %}