When updating a blog post widget you will have two options for "Which widget to use?"
When you select to use the "Blog Post Widget" it will use the values entered into the the Widget Settings. If you choose to use the "Global Widget" you will only be able to select it. You will not be able to update the "Global Widget" from a blog post.
To edit a global widget you can find the widget anywhere else outside the blog and update it under "Widget Settings"