I have used black plastic and it definitely works. Water looks black when ever the rest of the water is frozen. I have been doing this for many years and you just put enough decoys on the plastic to hold it down and keep it from blowing away. The best bet is to put most of the decoys around the edge to keep the wind from getting under it. I have also taken a few decoy weights 8 ounce or bigger along to help hold the edges down.