But now, as you can see, this blog is no longer using the Google+ as commenting system and back to Disqus because apparently Google+ comment form not showing up on an older post. I've found the explanation about this case. There's a warning on Google+ comments help page:
Changing your blog's URL will cause existing Google+ Comments to disappear. If you're considering moving to/from a custom domain or changing your blog's URL, you should do so before you enable Google+ Comments.Because this blog does not use a custom domain from the very beginning and then consequently Google+ comments does not appear on the articles posted when this blog was still using blogspot sub-domain. So if you ever change your blog address either a simple blogspot sub-domain change (e.g. example.blogspot.com changed to example2.blogspot.com) or a custom domain (e.g. example.blogspot.com changed to example.com) Google+ comments form and existing comments will not remain visible on your older blog post.
You can deactivate Google+ comments by visiting Blogger Dashboard, select Google+ tab, and then uncheck "Use Google+ Comments on this blog". Any comments made under Google+ will not appear on your blog and the default Blogger comment form will appear.