Old Parish Records of the Church of Scotland list
Duncan as the son of John Blair and Margaret McLuckie, baptized on August 18, 1751 in Balfron, Stirlingshire, Scotland.
The Associate Session of Balfron which contains baptisms of Buchlyvie from 1750 to 1752
shows Duncan Blair son of John Blair and Margaret McLuckie baptized on August 18, 1751 in the parish of Kippen.
The Buchlyvie Associate Session Baptisms (some marriages) 1752–1785
lists a Duncan Blair marrying a Mary Blair married in Buchlyvie on December 8, 1872.
There are also references to several children of Duncan and Mary Blair being baptized in Kippen:
||Duncan Blair - October 17, 1773
||William Blair - December 22, 1775
||John Blair - March 29, 1778
||Jean Blair - January 4, 1781
||Alexander Blair April 10, 1784
There is no way to know for certain that Duncan Blair, son of John Blair
and Margaret McLuckie, is the same Duncan Blair who married Mary Blair, but the dates and locations
certainly indicate this is not an unrealistic assumption.
None of the Robert Blair Letters mentions a brother Duncan.