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

[maiki]

That would be RSS Polling:

RSS Polling

Summary Allows you to automatically import content from RSS/ATOM feeds into your Discourse site as topics Install Guide This plugin is bundled with Discourse core. There is no need to install the plugin separately. This guide explains how to configure and use the RSS Polling plugin to automatically import content from RSS/ATOM feeds into your Discourse site as topics. Required user level: Administrator Summary This…

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@Alexander

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

That part is for embedding on the site: two sides of a coin integration.