Reply To: How to add a widget only on single blog pages?

This already exists with your theme. Just look for a widget called “After Entry”. If you take a look at the demo – http://demo.appfinite.net/squareone/2013/05/01/threaded-comments/ you’ll see the “Subscribe to our blog” part. That was added using the After Entry widget.

So all you would need to do is add a widget to the After Entry widget area and it will show up after your post.