Blackshirt was originally a series in Thriller - the illustrated prose magazine, not Thriller Picture Library - and a series of novels by Bruce Graeme. Blackshirt might have been so named because, although always dressed in a black shirt and trousers when going about his safecracker activities, he was always masked, either a domino, or more often, a black, full head covering affair. And as there were other books and characters called Black Mask, Graeme might have decided to go with a different name. Not certain about that.
The Blackshirt novels were continued in the '50's when Roderic Jeffries took the pen name of Roderic Graeme.
Bruce Graeme also wrote a few stories of Blackshirt's ancestor, Monsieur Blackshirt, taking the pen name of David Graeme. These stories were set in 16th century France.
The history of all this gets very complicated and I'm only using info. gleaned from conversations with the late Russell Aitken.
I forgot to add that the Blackshirt stories started around 1925, well before Il Duce.