The portrait on black paper is made with white crayons, sometimes with the addition of colorful ones (e.g. in case of clothes).
This is a unique drawing technique.
Making such a portrait consists of “reverse drawing”. – What I usually do on white paper with a pencil I have to leave as a black background, and everything that is not usually drawn with a pencil, e.g. the skin has to be covered with a layer of crayons.