With both custom domain publishing, and Google+ hosted comments, becoming more popular, it is inevitable that the two subjects would interact.
One key difference between Blogger hosted comments, and Google+ hosted comments, is how they associate with the host blog. Blogger hosted comments link to the blog - but Google+ hosted comments (aka Google+ stream posts) link to the URL.
Any time you change the URL of your blog, you will lose Google+ hosted comments - from your blog. The Google+ stream posts, however, are still out there - and they still link back to your blog.
Any existing Google+ stream posts, mentioning your blog, will link to the BlogSpot URL of your blog, after you migrate to a custom domain.
A properly executed custom domain migration will produce a BlogSpot URL redirected to the Domain URL. Any existing Google+ Stream posts will continue to link to the blog, because of the redirect.
Any existing Google+ comments, that are associated with your blog, are out there, in your readers Google+ streams.
All that you lose, with custom domain migration, is the ability to associate all of the different conversations about your blog, between the people conversing about your blog. Like URL changes in general, you lose a tiny bit of control - but you gain visibility.
An Important Update
If you did not use a Blogger / Google account when you Followed this blog, years ago, you are probably not Following now . During the past...