Dear Bronwyn,
I have no details of a John of this particular family in Kirkcudbright, and all the children of the family appear to have been born in Kirkcudbright itself. However, one daughter, Elizabeth (see 1841 census) is not listed in the various sources amongst the baptisms of David Blair and Margaret McMonies' children, though we know she existed.
The research is much easier now Ancestry have added the 1841-1861 Scottish census to their database, and I now know that 2 of the unmarried daughters (Jane and Charlotte) were living with their cousin, Jane Carson in 1861, which is particularly useful to add further proof of our Elizabeth being the same one as in 1841 with the Blairs, as she called herself Elizabeth Carson Blair at marriage (though I've yet to establish the link between the Carsons and the Blairs or McMonies). We also have Elizabeth's sister, Mary Blair, living with the family in Liverpool 1851-1871, being head of the family after the parents had died in the 1860's. She is listed amongst the baptisms of the family, which is how we located them originally.
If John is related to the family, I believe it's more likely that he descends from Hugh Blair's brother, John, who took Hugh and Nicky to court to annul their marriage in the 1740's. We know Hugh and Nicky had only one son, and he appears to have had just one son himself, who is the precentor & shoemaker, David.
John had more Blair descendants, amongst them his grandson, David, who was born in the manor house at Borgue and became a general practitioner but moved to Kirkcudbright town c1830. I have visited the house he owned, where he raised a large family. it's possible that one of his cousins whose family didn't inherit the Borgue estate may have been born locally. I have been puylling out some of my Blair data to see what I can find (which is why I hadn't replied as yet), and I'll let you know if I find a link to the Borgue/Kirkcudbright Blairs.
Impy