I agree that the song is based on an African American Spiritual. The tune is almost identical to the tune for a spiritual that's been passed down in my family since at least the 1850's. It's based on the parable in Matthew 25. Each verse has only one line, repeated five times as in "She'll Be Comin' 'round the Mountain When She Comes."
Verse 1) Who'll be the leader when the Bridegroom comes?
2) Six white horses when the Bridegroom comes.
3) Five of them were foolish when the Bridegroom comes.
4) Keep your lamps all trimmed and aburnin' when the Bridegroom comes.
5) Who'll be the leader when the Bridegroom Comes.