This is something I've been looking for for many years now. It would certainly improve my content production.

So far I have tried Mars Edit, and it works. But the app is $40 once your trial exprires.

Then there's Ulysses III who recently announced they will be adding blogging support. http://ulyssesapp.com/devblog/2014/01/23/this-ones-for-the-bloggers-announcing-ulysses-1-2/ The app is priced similarly.

I'd love to see a native Mac app that lets your write while keeping your content synched between online/offline. Performance is key here for me, as I have to managed thousands of articles and hundreds of drafts.