As was mentioned, part of the problem is that Lead Belly is considered a folk singer while other black musicians are considered blues singers. In fact all of them were playing any music they could get people to pay money to hear.

If you read Elijah Wald's well researched book Escaping the Delta: Robert Johnson and the Invention of the Blues you discover that most of the musicians we consider bluesmen today in fact performed a wide variety of music from pop and jazz standards to folk songs,d sukie jumps and even hillbilly music.

The reason we view them differently today is that Lomax was an academic ethnomusicologist trying to document folk traditions in the U.S. When he recorded Lead Belly, he was working in that context, not the commercial record industry.

The performers we regard as bluesmen are seen that way because they were recorded by commercial record producers who only recorded their blues numbers. All the other music they played was ignored in favor of the blues numbers. These commercial producers were selling “race” records to a predominately African-American market and they saw no value in recording the wide range of music the “bluesmen” actually played.

It took the folk music boom of the 1950s and '60s to interest people in Lead Belly's music and they soon discovered other blues players that seemed---because of the available recordings---to be much hipper than Lead Belly. When commercial interest in folk music subsided, so did interest in Lead Belly even though “the blues” escaped being labeled as folk music and reentered the popular music markets.

Lead Belly's legacy is really a victim of his times, the divergent goals of recordists and his untimely death from ALS.

      - Mark