Blur effects on a transparent background leaves a black ghosting effect If I add a simple UV sphere that emits white light, and in the compositor add a Fast Gaussian blur effect, then when viewing on a transparent background there is a noticeable black halo around the glow.

Any ideas what the correct way to do this is?

Update: It seems to have something to do with the horizon colour being picked up by the blur effect.  How do you prevent that from happening?  As a work around I can set the horizon colour to white, but I don't think that's the correct way to do it...

Here you can see with a red horizon colour, and changing the colour of the emitter to blue the effect is quite pronounced:
To ensure you get 'pure' transparency, you'll have to ensure the transparent world is used, instead of your (coloured) sky.

When using Blender Internal, make sure you have in the Render settings, under Shading panel Alpha set to Transparent



When using Cycles renderer, make sure you have in the Render settings, under Film panel the Transparency box checked.



Without these settings your alpha will be pre-multiplied with the sky color, which will look like your example.