He is talking about the blues, but not the music genre.
Even the blues is NOT unencumbered by influences from other "cultures". It is a true American form of music; as is jazz, ragtime, and yes, Rock & Roll.
Broadway musical theatre was heavily influenced by Gilbert & Sullivan ( Brits ), Offenbach ( French ), and to a lesser degree, Mozart ( German ); however, it developed into a PURE American form ( in the 1940s ), which others have copied.