The harmonica came to the United States via Germany, but the instrument itself was invented in China. Easy to come by, Americans, both black and white, were drawn to the harmonica. However, where whites considered it a children's toy, members of slave and free black communities incorporated the harmonica into their songs and field hollers, or work songs, which were the origins of the blues.
According to harmonica historian Tony Eyers of Harmonica Tunes, the harmonica was an important part of the blues genre in the 1920s and 1930s, when harmonica players John Lee "Sonny Boy" Williamson, Rice Miller (Sonny Boy Williamson II) and Little Walter rose in popularity through recordings and live performances. Because the harmonica was a relatively inexpensive instrument to buy, easy to learn and small enough to fit in a pocket, it became popular---and in some case synonymous---with so-called "hobos," unemployed men traveling who traveled the country and were often picked up the instrument along the way. Both black and white blues musicians employed the harmonica during this period.
Between 1940 and the early 1960s, the harmonica remained a staple of the blues band, even if the blues themselves were often relegated to gin joints and brothels, as rock and folk music dominated the popular music scene. Harmonica players found homes in the three styles of blues that had developed prior to and continued to be refined during this period: New Orleans blues, Kansas City blues and Delta blues. The harmonica playing for each style was fairly similar: wide vibrato, wailing tunes and the slipping from and between notes was common everywhere.
The harmonica declined in popularity as blues groups patterned themselves more on the instrumentation of rock bands during the 1960s, 1970s and 1980s, using electric guitar and bass rather than the traditional harmonica, upright bass or brass of earlier bands. While John Lennon, Mick Jagger and other stars all played the harmonica and used it from time to time, it was not a crucial element of rock in the way it was for the blues, even for blues-tinged rock.
However, a resurgence in traditional blues as seen in the bands Aerosmith and U2 led to the return of the blues harmonica in those bands' albums. Crossover music, mixing aesthetics of rock, country and other genres, has also included the frequent use of the blues harmonica.