When a SLR camera takes pictures, why are the pictures taken by black cloth gray?

Hello, my friend, this should be a white balance problem. For example, a piece of white paper, whether under outdoor strong light or indoor dark light, people's eyes know that it is white, because people have memory function, but the camera does not. It judges the color according to the light at that time, so it has a white balance setting. Under different lighting conditions, it is necessary to set different white balances, so that the camera can recognize colors and not regard black as gray.