I would say that the majority of the audience at the Chicago Blues and Jazz Festivals are blacks 30 and up. With younger it is a generational thing similiar to young whites not liking Sinatra or Big Bands. Tastes change.
The audience makeup of the blues festivals I've attended (mostly in California and Washington) has been quite different. ....mostly whites between 30 and 50. But then again, I've never attended one in Chicago.