As the season advances and the weather warms, it is important to remember that the plastic is still functioning. Clear plastic continues to warm the soil into the summer, perhaps making it too hot for the plants you are growing. In fact, a process called solarization uses clear plastic in the summer to heat the soil to a temperature which will kill disease organisms and weed seeds in the upper layers of the soil. Such heat is not desirable at the roots of our garden plants. Clear plastic may be used early in the spring to warm the soil for early planting of warm season vegetables but, especially here in New Mexico, it is important to remove the plastic or cover it with another mulch or reflective material as the weather warms.
Black plastic has the potential to burn whatever touches it.
The shading provided by the plants and the rapid heat loss from the thin film, the low heat holding capacity of the plastic film, allow us to use the black plastic as a mulching material without the excess soil heating that clear plastic provides. Mulch is an important tool for limiting weed problems in the garden and in conserving moisture as well as cooling the soil in mid-summer. However, it can be used to speed the heating of soil in the spring to allow earlier planting of plants which need warm soil. Proper choice of mulch material and then its proper use allows us to maximize the benefits. Yours is a good question.