What's the difference between a gray card and a black card for photography?

Black cards are generally used to balance the exposure of the sky and the ground when shooting landscapes, which is called dithering black cards. You can find related articles by online search.

White cards and gray cards are used to accurately restore white balance and photometry. For example, you can accurately expose gray photometry in a shooting environment with complex light, or you can accurately restore white balance by choosing photos taken in a custom white balance after shooting. Often used for portraits and still life shooting. For reference.