Apologies again - I had actually read my notes correctly the first time! Gertrude Blakeney married a different Col Robert Blakeney much later (28 May 1752).
For those who may be interested, the best source for this family that I have found is Burke's Irish Family Records (American Edition, 1976). It has more detail than Burke's Landed Gentry.
Burke's Irish Family Records states: "ROBERT BLAKENEY, of Castle Gallagh (alias Castle Blakeney), co Galway, Major in Sir Charles Coote's Regt of Drags, granted Gallagh, co Galway and lands in cos Mayo and Kilkenny by Cromwell for his mil services, said to have m 1st, Martha Bolton, niece of Sir Charles Corty. He m 2ndly, Susannah (d 29 Sept 1659, bur St Michael's Church Dublin), dau of Capt Edward Ormsby, of Tobervaddy, co Roscommon (see that family), and d (will dated 10 Feb 1658, pr 6 July 1660), leaving issue, 1 JOHN ..."
Of that John, it is said: "The eldest son, JOHN BLAKENEY, of Castle Blakeney (the family estates granted by Cromwell were confirmed by King Charles II by Letters Patent 5 Dec 1688), b 1649, educ Trin Coll Dublin (BA 1671) m 1671, Sarah, dau of Very Rev Dudley Persse, BD, of Roxborough, co Galway (see that family), a d 1691 (admon 18 April 1691), leaving issue, 1 ROBERT, of whom presently. 2 John, of Distington, Cumberland, Capt R Regt of Foot of Ireland, served throughout Marlborough's Wars, present at Battles of Bleheim and Malplaquet, s unm 21 May 1749, aged 68. 3 William, Corney 5th Drags 1 Susannah, m J Colpays, and had issue. 2 Mary, m Thomas Taylor, and had issue."
Again, apologies for any confusion. I think this shows that Robert Blakeney and Susannah Ormsby were not the parents of the Capt John in question.