Running a filled out form smart list does not provide an external referrer URL for that activity. Where do you see such a data point to call on in Dynamic lists and SC's? Unless you create a custom field that leverages a script to capture this and pass to lead source or elsewhere within the lead record, the inbound referring URL is only captured by the Original Referrer URL field. Yes, it is only stamped once - and when considering traffic sources of new leads that's all you need.
Ranae didn't say the first ever hit on the website, though, but rather "attribute the downloads to the blog post."
Also, to be clear, Original Referrer isn't captured by the form post. It's captured by Munchkin when the person's web session starts (i.e. it's the referrer of the very first Visit Web Page activity). So if I go to www.somenewssite.com on Day 0 and browse around www.example.com, then visit www.someotherblog.net/post999 on Day 10 and follow a CTA to fill out a form on www.example.com/landingpage.html, my Original Referrer will be www.somenewssite.com. The OR is useful, more often than not, but won't help attribute the download to the blog post.