[Sesse__]

Applying a blur filter to a premultiplied image is (very subtly) _correct_, at least if the goal is to emulate what happens if you used a physical lens to blur the same image. Not only does postmultiplied alpha mess up the correct pixel values (as the original post shows), but even without alpha, you get a “halo” that is weird and unphysical.

[im3w1l]

I think corysama had the right of it, you need to do all the operations in linear colors, i.e.

original image -> convert to linear color space -> multiply alpha -> blur -> composite -> possibly convert back to sRGB if needed

The wrong way I meant was

original image -> multiply alpha -> gamma-correct-blur -> composite

where gamma-correct-blur = convert to linear color space -> blur filter -> convert to sRGB