Following the outbreak of the First World War, Blacklock returned to England to rejoin his old regiment and quickly achieved promotion. He served as commanding officer of 10th Battalion of the King's Royal Rifle Corps from December 1915. He went on to be commander of the 182nd (2/1st Warwickshire) Brigade in 1916 and then commander of the 97th Brigade in 1917. He was wounded during the Battle of Guillemont in September 1916 and was awarded a Bar to his Distinguished Service Order for his actions. He was appointed General Officer Commanding 9th (Scottish) Division in March 1918 and General Officer Commanding 39th Division later that month. In this position he oversaw the 39th Division's robust defence of the Somme in the face of the German spring offensive. Following the Division's retirement from the front line, Blacklock was made General Officer Commanding 63rd (Royal Naval) Division in August 1918. His commanding officer, General Sir Charles Fergusson considered Blacklock to be the "finest commander he had ever seen." Blacklock also received the praise of other senior wartime leaders for his command style, including General Horne and Field Marshall Haig.