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 alph

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