I would think if you start with an ultra-white buttercream it should glow under a blacklight.
Additionally any neon colors tinted from ultra-white bc should at least look glow in the dark under a blacklight.
I'm not sure though that you'll be able to achieve icing so GITD that when you turn the lights off it glows. That would be along the lines of radioactive!
So my advice is if you've got any ultra-white bc try putting it under a blacklight...