ScreenFader--A-screen-fading-plugin-for-ImpactJS is a project mainly written in JavaScript, it's free.
Make your ImpactJS game screen fade to black or any other color you choose with this simple yet versatile screen fading plugin.
ImpactJS Screen Fader plugin
Official Page: http://impactjs.com/forums/code/screenfader-a-screen-fading-plugin