Well...if we're talking about the games in their unaltered forms then the naming conventions the programmers decided upon is fair enough to me as well.
However in terms of a
hack, that makes it my opportunity to tell the developer to slag off...

IMHO, I like the implicit nature of the logic I mentioned earlier. However I've taken to emulating the idea put forth in FF4 with the Diamond Armor/Shield/Helmet/Gauntlet/Ring which resists Lightning-elemental attack. I'm going to name Fire-Resistant equipment "Ruby" and Ice-Resistant equipment "Sapphire" in my hacks. To be a little more creative and to subtly suggest the usage. It's not obvious that the Diamond equipment resists Lightning but it is consistent in that behavior. That and it's interesting to picture Ruby Mail or a Sapphire Shield.
I've also decided upon making some armor for the paladin Holy (or I prefer "Light Elemental" as seen in the Seiken Densetsu series -- going to make that name change) resistant and all dark equipment for the dark knight darkness resistant. Which means I'm going to have to make monsters that attack with the darkness attribute (probably zombies, namely). That way the dark knight will be ineffective vs. zombies but they really won't hurt him either...meh. However that means sometimes the paladin would get his ass handed to him by zombies...oh well...that's just a challenge I guess

I'll have to make some enemies strike with the "Light" element early on to give the dark knight something to worry about

...and likewise, enemies that attack with "Darkness" later on for the Paladin...and Rosa. She's probably going to get some equipment that will resist holy as well. Like the "Light Robe", I mean shouldn't that resist Light Elemental attacks? I guess the Black Robe should logically resist darkness...
Lots of planning...and work to do in front of the hex editor...especially with enemies and magic to make the Darkness element actually useful...
For some reason I've always liked the idea of "Darkness" vs. "Light" in terms of "Elemental Power"...
Great discussion...this has given me a lot of ideas for making my hack(s) much more interesting.