As I stated earlier. Black Magic adds chaos and activity, thus destruction to their spells. White Magic is its opposite, bringing order and inactivity to its spells. Taken to extremes you have Spells like Foul which are essentially pure blasts of chaotic aether and spells like Holy which shock bodies into 'stasis'.
Magic in FFXIV is ultimately down to two things, it's elemental Aspect, and it's Astral/Umbral polarity. The origins of the word Astral/Umbral are mostly irrelevant at this point when they essentially mean "Active or Passive". Would it be better if the game just called it that from the start? Of course it would. But unfortunately they screwed this up back in 1.0 and now they're trying to make it work.
The elements prefer one charge or another but can ultimately exist within that spectrum. Hence what you said about how water can soothe and cleanse but also destroy. That too me would indicate that the element is charged in one way or another, water that heals and soothes can be argued to be more umbrally aligned since it's bringing order to something, whereas a destructive tidal wave can be argued to be more astrally aligned since it's full of force and activity.
The reason why black mages don't call forth destructive tidal waves and White mages can't heal with warm flames, from my perspective, is purely a game flavor thing. I don't see any reason with these rules set in place for that to be an impossibility. They did what they did to make White and Black mages completely distinct from one another in aesthetic and gameplay.
