A cyanotype, or blueprint, is an image made on a photo-sensitive (light-sensitive) surface, traditionally paper. It's similar to the printing paper of film (negative) photography. Fabric can sensitized with chemicals and images printed on it. For the foliage prints, the leaves are placed directly on the sensitized fabric and then exposed to light, producing a negative image (AKA photograms); i.e., the areas covered by foliage prevented the light from reaching the fabric.