config.yml

# Add this to your config.yml file
BlogPost:
  extension:
    -YoutubeIdBlogPostExtension

YoutubeIdBlogPostExtension.php

class YoutubeIdBlogPostExtension extends DataExtension {
    private static $db = array(
        'YoutubeId' => 'Varchar',
    );
    public function updateCMSFields(FieldList $fields) {
      $fields->addFieldToTab("Root.Main", new TextField('YoutubeId','Youtube Video Id'), "Content");
    }
}

This is a simple DataExtension class that adds a youtubeId field to the blogpost page.