Here's what I have found over the last few years of my esearch.
Robert Blair is actually Son of William McHallam & Annie Blain (maiden name) McHallam according to Census records 1881 for Kilmarnock here is the household list
William McHallam age 24 (Husband) Ann Blain McHallam age 24 (Wife) John McHallamage5 (Son) Catherine McHallam age 3 (daughter) William McHallamage 1 (son)
now again in 1891 same household more family :)
William McHallam age 35 (Husband) Mary H McHallam age 24 (new Wife?) John McHallamage15 (Son) Catherine McHallam age 13 (daughter) Robert B McHallamage 7(stepson?) William McHallamage 5 (son) Peter McHallamage 3 (son) now the ages dont all match up properly but are close enough depending their birthdates.
Now to go back even further to 1841 household in Coylton,Ayrshire there is a listing of John McHallum and Wife Catherine McHallum living with the Blair family along with Catherine's Mother Ann Reid there is also a notice of a child born to this couple named William McHallam in 1957 (note there was a name change/misspelling during all of this which lists William born to John McHallam and Catherine Reid McHallam). by ages would most likely make Him THE William McHallam of 1881 whom married Ann Blain in 1875 whom fathered my great-Grandfather Robert Blair McHallam.