What did the Blackshirts, Schutzstaffel, and Brownshirts have in common?

They were all Fascist paramilitary Organizations.

Increasing street violence in Europe post World War 1 between Left and Right led to the formation of paramilitary gangs.

The Blackshirts were Mussolini's thugs and the original Fascist paramilitary. The Blackshirts (Volunteer Militia for National Security) were formed immediately after World War 1 by disgruntled former soldiers to counter the Socialists. Other later organisations adopted the Blackshirt as their uniform notably the British Union of Fascists.

The Blackshirts were an important part of Mussiloni's "March on Rome" in 1922. The March was in effect a coup d'etat giving Mussolini power over the country.

The Brownshirts were the Nazi paramilitary. The SA "Sturmabteilung" were a creation of Ernst Rohm. Rohm, an early co-conspirator, helped Hitler out a great deal with financing and guns. His SA provided Hitler with muscle for Nazi party meetings. Hitler was not entirely happy with the arrangement since they were Rohm's SA not Hitler's.

The SA were a small organization to start but grew tremendously and eventually was country wide. As the German Government disbanded the Freicorps whole units came over to the SA intact.

Hitler to finally get a group of men that were truly his own organized the Stosstrupp Adolf Hitler later to become the SS (Schultzstaffel or Guard Detachment). These men were thoroughly indoctrinated into the Nazi creed.

The SA continued and got a shipment of brown uniforms that had been destined for German troops in East Africa during World War 1. Thus the Brownshirts got their name.

The SS grew to be a smaller but parallel organization. By the time the Nazi's were in power the SA had grown to 4-5 million members under Rohm. In 1934, a plot organized by Himmler, the leader of the SS and Herman Goring was to became known as "The Night of the Long Knives". Rohm was arrested and eventually shot in custody and leadership of the SA was systematically killed over one weekend.

The SA was never a force to trouble Hitler again.

https://en.wikipedia.org/wiki/Blitzkrieg:_From_the_Rise_of_Hitler_to_the_Fall_of_Dunkirk