Hi John,
Sharp, like all of the early collectors, was much more focussed on the songs than on the singers but, having studied his Appalachian collection, I'm coming to the conclusion that he began to take more interest in the singers as his work progressed. By his third visit, in 1918, he would quite often write half a page or more of notes about a given singer, appended to the relevant song in his 'fair copy' note book. Often there is more detail there than in his diary (if you've not accessed that yet, go to Sharp's diary online - I recommend selecting the transcription rather than deciphering his handwriting).

About Aunt Maria (whose surname is spelt 'Tombs' in Maud Karpeles autobiography and therefore presumably her diary), Sharp writes:

Aunt Maria is an old coloured woman, aged 85, who was a slave belonging to Mrs Coleman who freed her after the war and gave her the log cabin in which she now lives, which used to be the overseer's home.

Maybe that will give you a bit more to go on in tracing her? You can also search the diary by date (he visited Aunt M on May 22nd, 1918) to see what he got up to in Nellysford.

As to the song, it is one of many for which Sharp notated only a single verse. He seems to have done this in the case of songs for which he already had multiple versions (he was interested mainly in the tunes), but given that the verse he uses as an example for Aunt Maria's 'Barbara Allen' is rather garbled, it may be that she remembered only a fragment anyway. However, her tune is so good and so unusual that I've collated a set of verses from Sharp's other versions, in order to make a singable song (always the best kind of song, if you ask me).

Since this is going to be our first ever performance of many of these songs, it's a little early to be talking about a recording, but I would like that to happen one day.