From Wikipedia:

Each 'act' of the episode begins with a verse of a haunting folk song. The lyrics are as follows:

Fair was Elly Glover
Dark was Jess-Belle
Both they loved the same man
And both they loved him well.

By day she knew a woman's form
By night a witch's spell
For love of Billy Turner
Accursed was Jess-Belle.

A fitful night was spent by all
On Eagle's Rock did dwell
Strange things were seen by moonlight's fall
But none saw Jess-Belle.

Warm was Elly Glover
Cold dead was Jess-Belle
And husband would be Billy Ben
Of the one he loved so well.

No mention of who the singer was but music in the episode was by Van Cleave (who wrote the lyrics to the "folk song" that framed the episode). Episode was written by Earl Hamner Jr. of "The Waltons" fame.