On 25 June he was promoted to lieutenant in the 18th Battalion, 5th Brigade. On 18 August Blackett and his battalion arrived at ANZAC Cove during the Gallipoli Campaign. He was wounded in the right knee on 22 August, during the Battle of Hill 60, and on 18 September, again fell ill with dysentery. He was evacuated to the Blue Sisters Hospital in Malta by 22 September, and by 9 October was at the No.3 General Hospital in Wandsworth, London.