The blues grew from oral tradition, so not much is known about it or its players prior to the 1930s when recordings of field songs and folk music began. John Lee "Sonny Boy" Williamson is often recognized as the "father" of the blues harp and his recording career did not start until 1937. To confuse things, another blues harpist, Rice Miller, performed using the name Sonny Boy Williamson. History has designated them I and II respectively, and both remain revered and influential.
Little Walter broke new ground for the harmonica by coupling it with a dispatch microphone, or "bullet" mic, due to its shape. The harp/mic combination sat comfortably in the player's hands, and running through an amp, the harp could compete with guitar and add distortion to its sound. Chicago and Detroit blues increasingly featured the harmonica and leading players of the 1950s and '60s including Howlin' Wolf and Big Walter Horton. Jimmy Reed was the first guitarist to make an impact with a rack-mounted harp.
While there was a blues revival in the '60s, it was based much more on electric guitar -- and the popularity of the harmonica was relegated to more pure blues. Leading practitioners of that era include Paul Butterfield and James Cotton. Mick Jagger of the Rolling Stones brought blues style to the band, even as they moved away from their blues and R&B roots.
Kim Wilson of the Fabulous Thunderbirds, John Popper of Blues Traveller and solo artists Howard Levy and Chris Michalek have kept the harmonica in the blues idiom. Levy, in particular, has pushed the limits of blues harp, though he is not strictly a blues player.