"Black Peter" of Glenmoriston is likely not his gr grandfather. I have seen people with "Black Peter" as the husband of a Janet Grant (Dau of Aeneas Grant and Hannah Grant).
A few years ago my sister did a search on 'Black Peter" and found he was not married and died near Edinburgh. A minister recounted the tales of 1745 in a book called 'A lyon in Mourning'. Black Peter Grant is mentioned in this book.