I was surprised to see the TCE Summon Fey spell got reworked for the new PHB. I was very surprised to see they realized the Tricksy Fey needed the darkness buffed. I was even more surprised to see they did not understand why the darkness needed to be buffed. 🤦♂️
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.
If you have normal darkness everywhere, there isn’t a reason to use it, but you don’t always have darkness everywhere. In fact, you generally don’t.
Not all monsters with darkvision have access to light sources. Even if they do, they may need an action to use it or may be out of range. A torch or the light cantrip only has a 40’ range. If you collaborate on positioning with the caster, you can basically set yourself up to have advantage every turn thanks to the darkness, since as a ranged attacker you don’t have to stay within 40’ of your enemies.
Also, Gloom Stalkers can’t see through Darkness like Warlocks can, so this effect is useful to them in a way that the Darkness spell isn’t.
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. Sure, it still wouldn’t block even a cantrip, but it would block torches, lanterns, the sun, etc..
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. If you move one of those torches away and back, why would it suddenly pierce the magical darkness? If it wouldn’t, why would a new nonmagical light source?