As @Victor Castillo Torres said, you need to change the name of the tag you're loading, which will fix that aspect of your template tag. However, even though they are in different namespaces, I would still change the name of the context variable your tag returns just for sanity:
@register.inclusion_tag('featured_posts.html')
def featured_posts_list():
blog_posts = BlogPost.objects.published().filter(
featured_post=True).select_related("user")
return {'blog_posts': blog_posts}
Then in your template:
{{ blog_posts.count }}
<ul>
{% for blog_post in blog_posts %}
<li>{{ blog_post.title }} </li>
{% endfor %}
</ul>
And finally, in your main template:
{% load blog_extras keyword_tags i18n_future %}
...
{% featured_posts_list %}