The above WordPress onebox looks good on try:
It’s broken here on meta because http://sixprizes.com (WordPress blog) does not have SSL enabled, hence causing this error:
Mixed Content: The page at 'https://meta.discourse.org/t/onebox-broken-for-wordpress-4-4-sites/36765/2' was loaded over HTTPS, but requested an insecure resource 'http://sixprizes.com/2015/12/15/silver-surfer-back-to-back-runner-up-reports-a-fiery-standout-and-some-potential-plays/embed/'. This request has been blocked; the content must be served over HTTPS.
@AdamCapriola If your Discourse instance has SSL enabled than the accompanying WordPress blog should also have SSL enabled for embeds to work properly.