http://motherboard.vice.com/read/why-computers-have-trouble-with-blending-colors
I read this article, and think Affinity (all of them) need a more accurate blur, square brightness, average (or whatever else), then square root brightness. Simple compared to the other things, like photo's inpainting tool.
(If you are really curious, the other layer in the screenshot is a picture of a pine marten)