Here's some rough scheme I came up with (I never implemented it, though):

1. Use github pages to serve content.

2. Use github login to authenticate using just JS.

3. Use JS to implement rich text editor and other edit features.

4. When you're done with editing, your browser creates a commit and pushes it using GitHub API.

5. GitHub rebuilds your website and few seconds later your website reflects the changes. JavaScript with localStorage can reflect the changes instantly to improve editor experience.

6. Comments could be implemented with fork/push request. Of course that implies that your users are registered on GitHub, so may not be appropriate for every blog. Or just use external commenting system.