He served with the Coldstream Guards during World War I, was twice mentioned in dispatches, and was awarded the Military Cross for action in which he was involved on 15 September 1916.

He was deeply shaken by his war experience and, in coming to terms with it, with the loss of his brother and many of his contemporaries, acknowledged a debt to Freud's writings, which stimulated in him an interest in psychiatry.

A Guards officer and a sergeant who had laid the mines at that point walked into the minefield to bring back the corporal's body.

The differences in outlook, aspirations and in judgement of these two men during their partnership in office in the Eugenics Society led to tensions which have been ably traced by Richard Soloway.

Two prominent issues here were Darwin's reluctance to endorse Blacker's deep-rooted conviction that research and provision of contraception should be a major feature in the Eugenic Society's strategy to reduce the fecundity of the lower, less able classes and his disagreement with Blacker's aspiration to redirect more of the Eugenic Society's effort from education and propaganda to research and promotion of contraception."