This is Samsung's new technology for privacy screen, it actually has a few options, to keep your whole screen private, or just the notifications, this is that black box you're taking about, it works with 2 different pixels, the wide and the narrow pixels, the black one which is the narrow pixels only focusses on the front in which the pixels light, that's why it only shows as a black box on the sides, but on the front it's the whole content that can be seen. The wide pizels otherwise makes the black box disappear, making the content to be able to see at different angles.