Question

Why is light passing through straight edges of a window blind get curved?

Answer 1

The sun isn't a point source, it's a disk, so every point at the blind aperture projects a disk of light on the wall/floor/screen/whatever. Most of them overlap so you just see even illumination, but at the edges there is less overlap and some partial solar images are projected (the repeating concave pattern).

Corners come out rounded because they act as one-sided pinhole cameras, creating an image of the solar disk on one side and unfocused light on the other.

Answer 2

Thanks. Although I don't quite understand "one-sided pinhole camera".