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?
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 torches~~ nothing. Just illuminated by torchlight until a rule update does something other than make it bigger to fix the issue.
The rules text says it creates an area of darkness, and with your interpretation, it doesn’t, which means your interpretation is wrong. Yes, the ability could be written more clearly, but the logic for a reasonable way for it to function follows pretty cleanly. Your interpretation is not RAW or RAI.
There’s a reply on RPG StackExchange that covers a similar line of logic to what I wrote above.
Remember that Fifth Edition D&D is intentionally not written with the same exacting precision as games like M:tG. The game doesn’t have an explicit definition of magical darkness, but it’s pretty clear that the intent is for magical to trump mundane (when it comes to sources of light and darkness). Even the Specific Beats General section says that most of the exceptions to general rules are due to magic.