The Chicago Blues Festival was this past weekend, and I found a couple of articles that catch some of its spirit. Even though it was shortened to three days a few years ago, I think it's still the largest free festival going, and its Chicago heritage gives it some choice performers to pick from.
Here's a piece about the Saturday show.
And here's an article about the festival headliner Sunday night, Mavis Staples. She's not exactly a blues headliner, but she's a great singer and it was an interesting choice.