According to Kauffman's 1960 book "The Pennsylvania - Kentucky Rifle", William Blair is identified as a gunsmith on the 1823 tax list of Brothersvalley Township, Somerset County, Pennsylvania. I did not find a William Blair in transcripts of the 1820 census of Brothersvalley Township or the borough of Berlin. I did not find a William Blair in the Bedford County section of the transcript of the 1790 census of Pennsylvania (46 MB) or in transcripts of the 1800, 1810, or 1830 censuses of Somerset County. I also did not find an individual named William in the aforementioned census transcripts who had a last name that could easily be confused with Blair in cursive. I also did not find an individual named William Blair in the Berlin or Brothersvalley Township sections of the 1830 manuscript census records of Somerset County.
I did not find a reference to an 1800s gunsmith named William Blair in any other locale. The nearest gunsmith I know of with a name that could easily be confused with Blair in cursive was William Blain of Westmoreland County, which adjoins part of the western boundary of Somerset County. At least one book (Volume 1 of Hartzler's 2015 book "American Silver-Hilted, Revolutionary and Early Federal Swords") reports that the gunsmith William Blain worked in Brothersvalley Township, Somerset County before working in Donegal Township, Westmoreland County. I suspect that Hartzler's book is right, because I have searched diligently for documentary evidence regarding a gunsmith named William Blair without success.