Physical Principle White light reflects all light, which is naturally not hot, while black absorbs all light, which is very hot. Then why use black when avoiding light?

Because black absorbs light, black cloth is used to shade the light. After light is absorbed by black, it cannot pass through black cloth, etc. This is the principle of shading. As for the temperature, as long as you don't touch black objects in light (especially in the sun), you won't feel heat basically. If white can completely reflect light, of course white is the best. But often white will let a lot of light pass through when it is reflected, so it will not be able to avoid light.

Black absorbs light, and light can't pass through black to reach the area where we need to avoid light, and our goal is achieved. As for the heat, you can't feel it if you put it aside 10 cm.