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Black Knight is a specific character archetype about villainous knights or knight-like figures who are mysterious, powerful and feared. Monster Knight is about clearly nonhuman monsters who are also honorable warriors. What you've described doesn't sound like either trope. If they're common, interchangeable Mooks they aren't Black Knights, and if they're basically humans or close enough they aren't Monster Knights.
Edited by Theriocephalus
Focusing more on the fact that they are faceless and are probably wearing very similar types of armor, it sounds much closer to Faceless Goons who happen to have a Medieval European Lower Nobility aesthetic.
They are faceless, as any differentiation is only in armor color and details and is used to denote rank, and the humanity is questionable as the only instance of a Knight-type enemy in the work that speaks does so with Robo Speak. But thanks for the help! I can finally finish this page update.
Do nameless, faceless, questionably-human mooks identified as knights go under Black Knight regardless of armor color, or Monster Knight?