Google+ comments was integrated with Blogger platform, a long time ago, making it possible for Blogger users who have upgraded their Blogger profile to Google+, to use it as an alternative to the default commenting system on Blogger.
As it becomes more common on Blogger blogs, I keep seeing questions about disappearing comments after switching from .blogspot.com to a custom domain.
If you’re one of those that keep asking why your Google+ comments disappeared after changing to a custom domain, you have to be aware of the fact that changing your blog’s URL will cause existing Google+ Comments to disappear. Google stated this clearly when announcing the integration and you can also confirm it here.
Although, the comments, with the entire subsequent conversations, are still in Google+, in peoples Streams for them to see (and reply), they will disappear from beneath the blog posts on your blog once you change your Blog’s URL.
So, 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.
Recommended: How to Add Google+ Comments to Blogger Blog