Perhaps you could visit some local farmers markets and ask some flower vendors.

Black Eyed Susans bloom in my garden that time of year, but I'm much farther south. You might want to consider the fact that they push gobs of yellow pollen out and it might stain your bridesmaid's dresses. I purposely don't use them in bouquets for this reason. It sounds very pretty and like you're going for a more informal, country wedding look. You should definitely ask some local growers in your area what will be blooming at that time.

Joe Caputi, a member of the Association of Specialty Cut Flowers Growers, is located between Charlottesville and Richmond. I believe he is also a vendor at the Charlottesville Farmers Market.

BTW: The hybrid rudbeckia wouldn't have pollen.

Here is a link that might be useful: Joe's Website