Q. Why do nurseries use black plastic planters?

It's possible for black plastic to act as a sun collector which would cause the potting material to reach temperatures that would be harmful to the plants. If you have plants in dark-coloured plastic containers and notice that they are wilting more quickly than usual check to see that the plants are getting enough water and then relocate them to a position with more shade where there should be no problem with heat buildup. Plastic can be damaged easily by sunlight which can lead to fading and brittleness. A lot of plastic pots that are designed to be used outside have ultraviolet light inhibitors added to them so that they won't fade as quickly and will keep their flexibility for longer. This helps the pots survive longer.