As chief legal advisor to the Cabinet, Blackwell was involved in the prosecution of Roger Casement, and authorised the circulation of his disputed Black Diaries.
[10] As chief legal advisor to the Cabinet, Blackwell was involved in the prosecution of Roger Casement, and authorised the circulation of his disputed Black Diaries, advising the Cabinet in July 1916:[1][2] I see not the slightest objection to hanging Casement and afterwards giving as much publicity to the contents of his diary as decency permits so that at any rate the public in America and elsewhere may know what sort of man they are inclined to make a martyr of.
I am inclined to think that the rebellion itself and its results have given the impetus and that the situation today would have been much the same whether Pearse, Connolly and the rest had been shot or merely sent to Portland with a confident expectation of amnesty and early release.
The 'Daily Express' on three occasions has openly stated that he is a moral degenerate addicted to unmentionable offences and has cited his 'diaries’ in proof.In 1918, due to worries over the large number of firearms left in private hands following World War I, and the concern that they would be used by "savage or semi-civilised tribesmen in outlying parts of the British Empire" or by an "anarchist or intellectual malcontent of the great cities whose weapons are the bomb and the automatic pistol",[11][12][13] a Committee on Firearms Control was struck with Blackwell as its chairman.
Following his retirement, Blackwell's continued interest in legal matters was shown by his 1934 appointment as chairman of the statutory committee of Britain's Pharmaceutical Society, to which he had been elected an honorary member in 1928.