I'm unsure of how oneboxing is setup for blog posts to Discourse

[Alexander (Alexander Barrios)]

Do you mean the message that says “Originally posted on…”?

Embed Discourse comments on another website via Javascript

Discourse has the ability to embed the comments from a topic in a remote site using a Javascript API that creates an IFRAME. For an example of this in action, check out Coding Horror’s blog. The blog is run via Ghost but the comments are embedded from his Discourse forum. One important thing to note with this setup is that users have to navigate to your forum to post replies. This is intentional, as we feel that the posting interface on a Discourse forum is currently much richer than what we …

I’m not sure, but I think this is what you’re looking for