This is true not only for sand but also for wet paper or fabrics. It all has to do with the phenomenon of scattering , that is, the number of collisions that the light beam undergoes before surfacing again. The wet sand grains are covered by a thin film of water . This makes the change of direction of the light rays less and, therefore, more collisions occur than in the case of dry sand.
In fact, the light needs a minimum of six to go outside while it takes two in dry sand. With more collisions, the light loses intensity and therefore we see the wet sand darker.
