Toko HF Black is designed to repel dirt, but only in certain conditions is it recommended.
When snow is cold but also dirty and machined, Toko HF Black wax is generally not as necessary since HF Blue will make the base sufficiently hard and slippery to minimize the dry friction that occurs in these conditions.
When snow is clean and wet, suction is the main factor slowing skis down. This is easily minimized by a soft wax such as HF Yellow which makes the base very water repellent. However, when the snow is both wet and dirty, Toko HF Yellow will pick up dirt. The skis need a wax that will both repel dirt (a shear lubricant) and also be reasonably water repellent. The perfect wax for this is Toko HF Black. For this reason, Toko HF Black is most effective in snow that contains both free moisture and dirt. It can be used straight or mixed with another wax such as HF Yellow depending on conditions.
Ideally, JetStream and/or HelX should be applied over HF Black for top performance.