Far as I understand it, every element in FFXIV can appear in either the passive (umbral) or active (astral) form, some just trend more towards a certain polarity naturally. So while Fire is often found as it's astral form (explosive force), 'umbral fire' could theoretically exist too. In practice, that'd be the difference between a spell being packed with explosive power (our fire spells) versus something more akin to 'raise the internal temperature of a being until it cooks from the inside'. Similarly, umbral ice refers to the sheer cold of the ice spell, but astral ice would be like, idk, the rapid expansion of the ice as it freezes blasting apart whatever it's inside at the time? 'astral and umbral' in their pure unaspected forms refer to just the forces of stasis and entropy

Maybe that's just some fanmade BS I saw and internalised as 'actual lore' though, or maybe it was actual lore but got retconned idk, it'd be interesting to see it as a twist for a new job, where one might use elements in their astral form (BLM), this one instead uses them in their umbral form