Background Edit. However, this could all just be placeholders until the actual stats are set.
According to its entry data name and model name (ATX_Sten), it is based on the Sten gun, a WWII weapon that has previously appeared in Fallout Tactics.

The Black Knight is a weapon in Fallout 76.

It seems to be chambered to .308. According to its keywords, it will use hunting rifle animations. It shares a similar, albeit darker, appearance with the Brotherhood knight suit, indicating the wearer's status as an officer.

The Brotherhood officer suit is an underarmor in Fallout 76. Fallout 76 is a new Fallout game thoughtfully named after its predicted average review score.
The Black (K)night SMG Question I've seen varying reports alluding to an SMG called the "Black Night" (or Black Knight) that uses .308 ammunition, and I'm wondering if …

A one-piece, tight-fitting black suit worn by members of the Brotherhood of Steel. For Fallout 76 on the Xbox One, a GameFAQs message board topic titled "PSA Black knight atom shop skin".