What colors of clay makes black clay?

Theoretically, you could expect getting dark brown by mixing equal parts of cyan, magenta and yellow, but you will probably never succeed in obtaining true black, since it will be very difficult to mix them in the correct proportions, because available pigments are not ideal cyan, magenta or yellow.

Try a 1:1:1 mix, then observe the obtained brown; it should reveal a predominant hue that is the closest from one of your starting 3 colors, so add very few of the 2 others.