Why isn't it grey?

why am I getting purple and not grey

border.ffxml

My guess:
In FF, the color information is still remembered as part of an image -- even when the alpha channel = 0. When you blur some image with transparency, it seems to also blur the alpha channel too, and then, thereby, exposes some of the colors on the other channels (because the alpha/transparency parts got blurred/reduced).

Maybe look inside my attached filter to see better what I'm trying to describe...

(And by the way, I'm not saying this is how it "should" be -- I'm just describing how it seems to be.)



So: Constantin's advice is sort of the way I would work with this kind of stuff.



blur and alpha0.ffxml

Short comment: ssamm is right