It is doubtful that she's the daughter of Big Billy Stacy and Rebecca Blankenship as they had a daughter, Martha Jane Stacy, born in Tazewell, Virginia in February 1857 and a daughter Rachel Stacy, born in Tazewll, Virginia in 1858.
"Virginia Births and Christenings, 1584-1917," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:X5XR-WXS : 5 December 2014), Parthema Stacy, 23 Nov 1857; citing Elizabeth City County, Virginia, reference 553; FHL microfilm 2,046,929. gives her parents as William Stacy and Mary Jane Stacy, but the birth was in a different county (Elizabeth City) in the same state. Though it is well known that Big Billy Stacy had two wives and a mistress, a Mary Jane doesn't qualify. Was their another William Stacy in Virginia who married a Mary Jane who could be Parthema's father?