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.
But you’d have to go outside the walls to get to the turrets. And you’re in a situation where you have someone who can cast 8th level spells. I’m not sure this advice is sound. 🤨
Well, technically it doesn’t say your connecting walls have to be straight, just 80 feet long. Not exactly sure how you’re going to make them connect up into a hexagon or star out of “four turrets with square bases, each one 20 feet on a side and 30 feet tall, with one turret on each corner”, but if you’ve got a diagram I’d love to see it. 😆
Your RPG StackExchange answer admits near the end “[…]as written, Tricksy Darkness has no effect because it doesn’t explicitly mention preventing nonmagical illumination, it’s logical to assume that it was intended to do something[…]“ The assumption that it should do something is the basis for the author’s claim that it blocks non-magical light.