Summary: The absorbing surface gains its lightness or darkness from the fraction of light rays that are absorbed when they hit it. When present the liquid surface reflects a certain fraction of exiting light rays back down to the material, giving them multiple chances of being absorbed (one chance for every time they are reflected back down). Chances are involved because the material's surface is rough, reflecting the light that is not absorbed back out in random directions, some of which strike the back of the fluid-air interface in a way that sends them back down to the material.