One possibility given her age shown at death, again from Ancestry's London Parish Registers:

Born 27 November, baptised 25 December 1778 at St Sepulchre Holborn Margt BLACKLOCK daughter of John & Mary

One sibling: born 14 November, baptised 8 December 1776 at St Sepulchre Mary BLACKLOCK daughter of John & Mary

Can't find a marriage for John BLACKLOCK & Mary, I'm afraid.
Janet