This is very weird but Umbral Blades gets no Fire tag for specing Exploding Blades. I’m almost positive the explosions count as a fire skill for purposes of things like Soulfire or Calamity, as well as fire damage modifier. After a lot testing I’m almost positive this is the case. Yet, the tooltip doesn’t shown that this is the case. This might need some clarifying either on the Exploding Blades node or just by adding the Fire tag after taking the skill spec.
I realize this is probably only because the initial hits are unaffected, but that still is kind of unclear.