I must respectfully disagree about reblacking as even 20-30 years is not long enough to know the effect over 100-200 years. Still, I don't know how we prove it one way or the other and there probably is not any interest in the right sector to get to a real answer. I just choose to be cautious.
Caution is good, but some balance is in order.
Let us consider that at least some blackening processes may actually have a stabilizing effect. We do know that faded hard rubber has a surface that has become porous, with a high free sulfur content. Expose it to moisture, and you end up with sulfuric acid. This is certainly not good for pen parts of metal and other plastics, and it probably is not a good thing to have in direct contact with the hard rubber layers below the faded surface. So there is a good argument for treating the surface so that active acidification is stopped, or at least drastically slowed. This is one context where applying wax is probably a good idea. I would also recommend applying wax as a protective layer before working on hard rubber pen parts where the exteriors are likely to be splashed with water (there's no sense in actually immersing caps and barrels or putting them under running water: stick to test-tube brushes and cotton swabs for cleaning them out).
Also, we are not talking about a 20-30 year history -- unless you are looking only at the world of pen collecting. If you do the research -- and I have, necessarily, being now in the hard rubber manufacturing business -- hard rubber has been used in industry for over 150 years, with a voluminous technical literature. One of its main uses was as a chemical-resistant coating for metal vats and other containers. It was soon discovered what chemicals could be used with these coatings -- and to put it succinctly, the few that weren't compatible aren't the sort of things to be found in the compounds presently used by most pen reblackers.
By the way, to address the discussion concerning cream vs film abrasives on mixed-color hard rubber:
Using a cream-type abrasive is often fine, as long as the fading doesn't go too deep. But if there is deep deterioration of the surface, and a significant difference in hardness between the colors, you will then get differential removal of the surface when using a cream -- Ripple surfaces that are now really rippled, like a washboard.
Under these circumstances, the only way to keep the surface flat while removing the deteriorated surface skin, is to use abrasive film, preferably backed up with a sanding block. It's the same in woodworking, where the hardness of the material isn't consistent all the way through.