Use Advanced Custom Fields – replace line of code that hardcodes the tag into the template

Display post by Using ONE Tag

Display post by tag id in same way. You just need to change the tag parameter name. This will display post which are tagged with 13 tag id. Here 13 is tag ids which is used just for example to explain in better way.

$tag_query = new WP_Query( array( 'tag_id' => '13' ) );

---

Display posts from either tag

To display posts from either tag you need to use tag__in. This will return posts from either tag id.

$tag_query = new WP_Query( array( 'tag__in' => '13, 27' ) );

---

Now, you can try below code to fix your issue.

<?php $tag_query = new WP_Query(array('tag__in' => $blog_tag)); ?>