Mysterious Anonymous Comment that won't go away

From time to time, I’ll get a comment notification from “Anonymous”… it won’t delete, and it won’t go to spam. Sometimes it disappears after a few days, sometimes it’s there for weeks, and sometimes it takes an update to kill it.

I have one on there now from 7/22/2015 that won’t die. I’ve blacklisted the IP, but the comment is still there.

Any help would be appreciated. It’s driving my OCD berzerk.