Blog arithmetic captcha has gone missing

The blog comments used to ask you to add two numbers while submitting your comment. That didn't remove all spam, but it sure did cut down on it.

In the past week that feature somehow got disabled. The result is that we get about one spam comment per minute, which is a rate that I can't keep up with. So I've disabled comments on the blog for now on the permissions table: https://blog.torproject.org/admin/user/access

But it would be great to re-enable them, and some sort of rate limiting or captcha or something system seems needed for that.

Is there a drupal package we need to install? Or is it installed but configured to be disabled?