Considering the Guardian Games theme, I would assume it is probably Rivalry Stoic.
Unfortunately, there aren't any all black shaders to my knowledge, but there are a few mixed shaders with good looking black palettes in them: Amethyst Veil, Carminica, Bloody Tooth, Rivalry Blacksand (I think), New Age Black Armory, and Valkyrie Zero to name a few.
I myself have often used a mixture of Bloody Tooth, Amethyst Veil, and Carminica.
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Theres also Metro Shift on certain armors [spoiler]same with the Trials of the Nine shaders too[/spoiler]
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CARMINICA HAS GOTTA BE MY FAVORITE SHADER
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Its definitely not Rivalry Stoic, that shader has some white and like maroon-ish tones in it