Possible 1841 census for Ann Low
https://www.freecen.org.uk/search_records/5a149f10f4040b9d6e9bf41f/anne-low-1841-fife-dysart-1822-?locale=en
AND
1851 census backstreet Pathhead, Dysart
Ann Law 59 weaveress, born Cockpen Edinburgh
Margaret Law 16 daughter weaveress, born Dysart
William Black 8 grandson, born Dysart
Possible birth for Margaret LOW born 26/8/1834, baptised 8/ 5 /1836 in Dysart - daughter of John Low and Anne Honeyman
If that's the correct Low family, then it looks like our Ann Low was married previously - the grandson William Black is the son of William Black tin plate worker and Ann Low. William Black and Ann Low were married on 17 July 1841 in Dysart. Their son William was born about 1843, although there is no baptism that I can see.
William Black junior married Mary Ann Thomson in Tain in 1870. He was a stone mason and then superintendent of the water works at Edinburgh. He died in 1911.
On his marriage record, his parents are noted as William Black, tin plate worker (dec) and Ann Low (dec).
So, time for a bit of speculation
If this is the right Ann Low, did her first husband die and then she took up with James Ramsay? If that's the case, then it seems probable that James Ramsay, aged 10, who appears with the family in 1851, is actually the son of James Ramsay's first wife Catherine Wilson, who died in 1841. And Ann left her son, William Black to be raised by his grandmother?
Still doesn't help with finding a death for Ann Low. She was still alive in 1866 when her son David died in Alyth. If we are to believe William Black's marriage registration in 1870, she was dead by then. Her mother, Ann Low (nee Honeyman) died in 1870 in Leslie, Fife.
Ruth