Currently we were only oneboxing WordPress links with ../yyyy/mm/dd/.. permalinks, just added support for ../yyyy/mm/.. permalinks too:
https://github.com/discourse/onebox/commit/f66b171cc79e067022d457d62a0883b33383c9c2
Posts from http://blog.discourse.org are still not getting oneboxed because we are not providing oEmbed endpoint, will modify our WordPress theme to include oEmbed endpoint for posts.