Burl, thank you for that tale which goes some way to explaining the reason for the enduring popularity of this ballad. It does seem to me as well that a lot of the power of the song comes from its obscurity - not everything is explained clearly - so that it has a richness that allows many different interpretations, while still keeping an air of mystery about it.
However I'd like to pay a belated tribute to one of the finest singers of this song: Phoebe Smith, the Traveller singer from Woodbridge in Suffolk. She died, aged 88, on 8 November 2001 and with her death passed a great tradition of singing. Her voice can be heard on some of the Topic Voice of the People series of CDs, but for a spell-binding 11 minute rendition of Barbara Allen, listen to the Veteran CD VT136CD The Yellow Handkerchief.