Source for son Thomas, Ancestry.com, Ireland, Births and Baptisms, 1620-1911.
Also, Cousin Margaret Agnew, mentioned in a personal e-mail ‏(3 Jan 2013)‏ to author, that John and Margaret ‏(Agnew)‏ Blair had sons who spent time with their relative, David E. Agnew, in Boston, MA, but returned to Ireland.
Yes, brothers Daniel and James Blair came to US in March, 1912, and then departed NY, NY, and returned to Londonderry via Glasgow, Scotland, 6 Oct 1913. Daniel ‏(26)‏, James ‏(28)‏, and their sister, Jane Blair ‏(20)‏, were on this return trip together
Sources: Ancestry.com, Boston Passsenger and Crew Lists, 1820-1943; Border Crossings: to U.S., 1895-1954; UK Incoming Passenger Lists, 1878-1960, listed Daniel, James and Jane's last country of residence as USA. Daniel and James planned to return, but Jane planned to stay in Ireland.