Magical darkness behaves exactly like normal darkness in 2014 5e and 2024 5e, unless other rules modify its behavior. By default, it doesn’t block line of sight, darkvision, magical light, or even non-magical light unless something specifically says it does, like in the darkness spell for instance.

That all said, Tricksy wouldn’t do anything if it didn’t block nonmagical illumination, so it’s reasonable to run it as though it does.

That’s my point. It never says it blocks even non-magical illumination, so therefore, does functionally nothing.

And running it as though it doesn’t block nonmagical darkness results in nonsensical behavior. You’re in a torchlit chamber and use the ability - now there’s a cube of darkness, blocking the light of all four nonmagical torches.

You’re in a torchlit chamber and use the ability - now there’s a cube of darkness, blocking the light of all four nonmagical torches nothing. Just illuminated by torchlight until a rule update does something other than make it bigger to fix the issue.

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.