A change has been made to the publishing process of a blog post. When a blog post is "published" the Add Post screen will be locked. This will prevent edits from being made to a live blog post. A post must be "unpublished" before any edits can be made. Once the edits are made the post can be republished.
This change is the same change recently made to the calendar. The complexities of updating live events and posts and what happens behind the scenes has made this change necessary.