Best way I've seen is color it black in the albedo, make it 100% rough in the smoothness mask, and 0% metal. That should prevent any light or specular getting to it. It's a fake hole - it's only going to look SO good.
Yes, entirely metal with black albedo makes more sense. 0% metal will be 4% reflective. Metal value of 1 and albedo value of 0 and then the gloss/roughness doesn't matter. Though either way will work fine.
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If it's 0% metal, you'll still get a little specular. But setting it to metal will use the black diffuse as the specular, and it'll just be a void.