When snow is cold but also dirty and machined, a black wax is generally not as necessary as 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 which is easily minimized by a soft wax such as HF Yellow which will
make the base very water repellent.
However, when the snow is both wet and dirty, there is a problem. Address the wet with HF Yellow and the skis will pick up dirt. The dirt needs to be addressed with a wax that
will repel dirt (a shear lubricant) that is also reasonably water repellent. The perfect wax for this is HF Black. For this reason, 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.
Obviously on top of the HF Black, ideally the JetStream wax and/or HelX wax of the day would be applied.