A varnish maker can give varnish a matte appearance by adding colloidal silica (Cab-o-sil). This gives a matte surface in two ways. One, it gives a roughness to the surface such that light is reflected in different directions from the surface (look up "Diffuse Reflection" in Wikipedia for a diagram). Two light rays that penetrate the surface of the varnish will encounter silica particles that have different index of refraction than that of the varnish resin and now have another new surface to reflect off. Hence, diffuse reflection from the bulk of the resin layer results in a matte finish. IMHO the use of wax would be highly unlikely.

Silica doesn't absorb UV or reflect it in an appreciable manner.

UV protection is afforded to varnishes and other materials in a much different manner. It has been observed by many that organic compounds with double bonds absorb UV electromagnetic radiation (light) and reemit it at longer wavelengths. The most readily observable example of this is graphite added to rubber tires. Leave a rubber balloon out in the direct sunlight for a summer and it will turn into crumbly junk in a few months. But an old tire can be used as a bumper on a wharf in the direct sunlight for eons without losing much in structural integrity.

The black would be unacceptable in appearance in a great many applications but fortunately there are a great many smaller, colorless molecules that are good UV absorbers.

Metal flake would make a good UV protectant by reflecting the UV light from the material however it would also reflect a lot of the visible light also. Consider a Doretto chip bag.