I have a strong suspicion that your Margaret Hartgrove could be a sister to James Hartgrove who died bef. 20 Nov. 1773 in Augusta County, VA, and MAY have been the Hartgrove that married Elizabeth Blair, daughter of William Blair and Elizabeth Downey.
I'm still researching this family, thought I'd pass along this clue.
There is a William Hartgrove that I suspect MAY be the father of James and possibly Margaret, that was listed in the following record in Augusta County:
Vol. 1 - APRIL AND JUNE, 1746. -John Trimble vs. Wm. Hartgrove.--Bond 3d February, 1738-9. Note.-- Lives at ye forks of ( ) and married to ye Widow Barnet.