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. 🤦♂️
It never actually specifies where the heavily obscured area is. But I think it makes more sense to interpret it as only the area in darkness is heavily obscured. After all, the area behind it depends on the observer, and there's nothing about that what's heavily obscured by darkness depends on where you're standing.
On the other hand, if this just makes everything in that area dark, but still lets light pass through it, you'd still be able to see silhouettes of anyone standing in it, which really doesn't seem like it's heavily obscuring them.