Q's advice is sound - you will probably want to pick specific songs common to black and white artists from the 1920s and 1930s and attempt to tease out the elements that flowed from white tradition to black musicians. No easy task, but a worthy topic.

You will need some background on the topic. The hillbilly and race recordings of the early twentieth century reflect over 200 years of hybridization and cross-fertization between blacks and whites in North America (and the Caribbean, as well). Bill Malone in the introductory chapter of Southern Music, American Music discusses the difficulty in determing the 'racial' origin of many American folk songs. Secular and religious music demonstrate this interaction, as do folk and popular forms. A single song may weave its way back and forth across the color line, from the stage to the work-camp and back, and may parody a hymn or recast a secular theme in religious terms.

Lyrics and melodies common to both black and white tradition found their way onto race and hillbilly records. You will need to look for demonstrably 'white' material that has been translated into the African-American idiom (as clumsy as that sounds, I don't know how else to say it). "St. James Infirmary" (mentioned previously) is a good example of this. "Cotton-Eyed Joe" is an example of a song that, while likely of African-American origin, is melodically related to British-Irish music.

You may want to get a hold of the article "The Blues Ballad and the Genesis of Style in Traditional Narrative Song," by D.K. Wilgus and Eleanor Long (Narrative Folksong: New Directions. Boulder Colorado: Westview Press, 1985). Wilgus and Long sketch the outlines of the American Blues Ballad, which "does not so much narrates the events of a story as it celebrates them." Assuming a degree of familiarity on the part of the listener with the basic events of a story, the blues ballad uses allusion and poetic affect, and plays loose with chronology. Noting that the blues ballad is found both in white and black tradition and that its form coalesced in the United States in the second half of the nineteenth century, the authors argue that songs with these characteristics are found in Irish tradition, and that these forms can be traced to the early Middle Ages. While they don't quite make a direct connection between Irish forms and the American blues ballad, they suggest the possibility of a connection (without discounting possible African antecedents).

Also, for reference, you will want to obtain a copy of G. Malcolm Laws' Native American Balladry (way overdue for a reprint).