First, black is the absence of visible light and white is a uniform distribution of visible light. It has nothing to do with IR or UV, which we don't see. But for the purposes of this post I'll assume your Nikon lens absorbs only visible and IR and your Canon lens absorbs only UV.

While it is true that a UV photon has more energy than an IR photon (and is therefore "hotter" ), there are a lot more IR photon in sunlight at the earth's surface. UV is fairly strong in space, but the very energetics inherent in the UV photons make them susceptible to absorption in the atmosphere. And I'm not talking about the air around you, but the atmosphere 100 miles up. The air is pretty thin up there, but it's quite effective at blocking most of the solar UV. Even more effective is the ozone layer around 50 miles up.

Here are two illustrations showing the solar spectrum and the transparency of the atmosphere at the surface. They are from the Handbook of Geophysics and the Space Environment, Air Force Geophysics Laboratory, 1985. Note the rapid fall in the intensity of the solar spectrum in the UV (to the left of the visible spectrum) and note further that the graph uses a logarithmic scale. Also, the transparency of the atmosphere is small in the UV while the IR has a number of transparent bands.

I always thought the fluorite was used as a lens element, not a coating.
This from Canon's website: Link Here

"Fluorite, which boasts a very low dispersion of light, is capable of combatting the residual aberration that standard optical glass fails to eliminate. Canon succeeded in artificially creating crystal fluorite in the 1960s, producing the first interchangeable SLR lenses with fluorite elements. In the 1970s, Canon achieved the first UD (Ultra Low Dispersion) lens elements incorporating low-dispersion optical glass. This technology was further improved to create Super UD lenses in the 1990s. A combination of fluorite, UD and Super UD elements are used in many of today's super-telephoto L series lenses, telephoto zooms and wide-angle lenses."

Fluorite coatings on lenses have been around for decades. The coating is on the glass, not the lens body. The purpose is to provide a graded index of refraction interface between the glass and the air that reduces reflections (and flare). Has nothing to do with absorbing heat.

Newer lenses use newer materials for the coatings.

And as far as starting something, don't worry. Some of these threads take on a life of their own, reaching far afield from the original topic. Guess this is one of them.

I suppose people on this forum like to practice their typing skills.