Wild Petunia (Ruellia humilis) is one of the longest blooming of native plants, producing lavender-blue Petunia-like flowers from mid summer through fall. Flowers are borne atop leafy stems standing 18"-24" tall. It is late to break dormancy in the spring so it's a good idea to mark your plant so as not to mistake it for dead when it doesn't emerge in the May garden.
Ruellia is a Midwestern dry prairie native so is well adapted to dry soil and will tolerate light conditions ranging from full sun to partial shade. It self sows generously when it is happy in its siting.