Darken is a transparency type in the Transparency Tool. It is useful for making underlying objects darker. Usually you would use it on a separate object to modify the color of underlaying objects (that is, you would use an object as a filter). Darken is a grayscale version of Stained Glass transparency type.

The color of the object with Lighten applied:

White - no effect on the colors of underlying objects.

Gray & Black - darkens the colors of underlying objects.

Other colors - convert the color to the equivalent shade of gray. (Yellow gives light gray, Red dark gray.) Then darken the colors of underlying objects.