Fun fact! One way mirrors are actually bi-directional. You get the one way effect by having one side being much dimmer than the other side.
The same amount of light passes through from each side. But the side that's brighter basically can't see the dim light coming through the mirror from the dim side.
It's like standing outside on a very sunny day, looking inside a building, but just seeing a dark hole in the building... It's so bright, your eyes are adapted for the bright light, so even though you know there's a light on inside the building, you can't see it. Same principle
The reflection of the bright space behind the viewer overpowers the image of the dim space on the other side.
This is how it’s not just the “dark hole” effect you’re referring to. It’s a dark hole with a bright image superimposed over it. Your eyes can’t adapt to the dark hole because of the reflected image.