I called Yahoo again. I pestered them and found out that there WAS a change on their side of things. I was able to resolve the issue.then later
The change they made was that they now require the entire email address as the user name (email@example.com) whenever you're going FTP something. In Blogger, I made this change under: Settings - Publishing - FTP username.
If you have a blog published to a Yahoo server, using FTP, and your publishing has stopped working this week, maybe this is why you're here.
Check the settings for your blog, and contact the Yahoo support team. Verify the change, then update your settings.