No reason it wouldn't block it, not sure what OP is on about

Darkness creates a heavily obscured area.

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.

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.