I'll see what I can dig up, Hanna, but I haven't done much specific work on them yet. I may have his parents' names (or at least one parent's name), and I might have the name of a sister somewhere...he was married and had children. Will have to hunt around to see what's on file here.
I realize this thread is a bit old, but should anyone else be interested:
David Blair was one of four children born to his parents. David's father, along with an uncle, was a military officer. Two of David's siblings were born in India, while he and another sibling were born in the UK. I believe one of David's siblings died in childhood.
David was married twice; his second wife outlived him and never remarried. David himself outlived his siblings. For some time his brother (he apparently followed in the footsteps of their father and entered the military) lived with him in David's Middlesex home. Blair had a daughter named Elizabeth Nancy. I have seen mention of a son as well, but I have not been able to locate his name.