I think there is nothing wrong with Scathe, it certainly has its place in the BLM kit. Extending Astral Fire or Umbral Ice with it seems like a pointless change, since Fire1/Ice1 already serve that purpose, and quite well too.

If BLM would need any kind of change, then Aether Manipulation allowing you to jump on objects/non-hostile NPC's outside of Party and maybe even shareable Leylines at increased Cooldown.