Black Light applies visual effects to the screen by changing its gamma curve. You can invert colors, darken the screen beyond what's usually allowed, apply a color filter, or apply the 16-235 luminance gamut expected by some HDTVs.
What's new in Black Light
Resetting gamma curve more proactively on quit, instead of counting on macOS to do it for us. This does not change the behavior of the app but resolves a somewhat nebulous issue with video capture and/or playback with certain applications on particular hardware configurations.