You probably didn't know that because it isn't true.
The Black Hand was a secret terrorist organization inside the Serbian Army. As such, it is likely most if not all of its members were Orthodox Christians.
Its motivations were, however, nationalistic, not religious.
BTW, this is a good example of the perils of making assumptions about dates and numbers and such. Sometimes a date is just a date, or an attack takes place on a specific date because that's when it was possible to make the attack.
BTW, many organizations down thru the years have used the Black Hand as a symbol.
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