But Umbral Sight works with normal darkness too, and if it doesn't prevent magical or non-magical light from illuminating it, people don't have to rely on darkvision to see them in that magical darkness (if it's illuminated by anything). At least the Hallow darkness effect prevents illumination, so that does work for Gloom stalkers, but this is quite literally useless.
That’s my point. It never says it blocks even non-magical illumination, so therefore, does functionally nothing.
You’re in a torchlit chamber and use the ability - now there’s a cube of darkness, blocking
the light of all four nonmagical torchesnothing. Just illuminated by torchlight until a rule update does something other than make it bigger to fix the issue.