Curing of concrete is a function of two things: moisture and temperature. The more moisture present and the higher the temperature, the faster the concrete cures. Less moisture and a lower temperature means less curing takes place. Covering fresh concrete with plastic is designed to serve two purposes: to keep as much moisture present for curing, and to minimize temperature release.
The difference between clear and black plastic is as simple as wearing a white or black T-shirt in the sun. The black T-shirt makes the wearer much warmer. The same is true for black plastic. Under identical conditions, black plastic generates a higher temperature for curing than clear plastic does. A higher temperature moves the hydration reaction along faster, which typically means that more water should be present during curing.
So, you could use either one, black or clear. Just understand that by using black plastic, the temperatures underneath could be much higher than you are used to with clear plastic. Take appropriate steps to ensure that adequate moisture is present during curing. -- Dr. Richard S. Szecsy, Ph.D., P.E.