What you are looking for is a Circular Polarized Filter. It is made by a plethora of manufacturers and comes in a variety of sizes. One of which should screw onto the threads of your camera lens.
Basically the Circular Polarized Filter, often referred as a C-P-L filter, reduces light that is reflected off a surface. You rotate the front part of the filter to minimize the reflected light. This is good to "see" below a water surface or to take the shine off leaves, etc.
It will also "darken" the sky slightly, depending in which direction you are pointing the camera with respect to which way the sun is shining.
It is a very good thing to have whenever you wish to reduce reflected light. They are not very expensive.