I have finally found the setting that enables Google+ comments. Unfortunately, "enable" is a word that does not adequately express the strength of the action. In fact, "enabling" Google+ comments replaces the alternative comment mechanism that this blog used to have. That setting is now turned off - you no longer need to have a Google+ account to make a comment.
I apologize for the frustration that this has caused.
(and now I expect some comments on this post...)