From Encyclopaedia Britannica:
The first squads—each of which was called Squadre d’Azione (“Action Squad”)—were organized in March 1919 to destroy the political and economic organizations of socialists. By the end of 1920 the Blackshirts were attacking and destroying the organizations not only of socialists but also of communists, republicans, Catholics, trade unionists, and those in cooperatives, and hundreds of people were killed as the Fascist squads expanded in number. A Fascist convention in Naples on October 24, 1922, provided the pretext for the concentration of armed Blackshirts from all over the country for the famous March on Rome that put Mussolini into power.
Early the next year, on February 1, 1923, the private Blackshirts were officially transformed into a national militia, the Voluntary Fascist Militia for National Security. The black shirt was worn not only by these military Fascists but also by other Fascists and their sympathizers, especially on patriotic occasions. With the fall of Mussolini in 1943, however, the black shirt and the Blackshirts fell into disgrace.