I think I've figured it out.
Last year we moved from WP to a blog that better integrates with our CMS.
I left the /blog/ folder in tact, yet deleted most of it's contents. I forwarded all of the articles, to the new pages. I didn't forward the /blog/ to the new blog base, as my redirects kept screwing things up.
Google has obviously continued to visit the URL and noticed the older version of WP that it had been running.
I am going to figure out the 301 now.