It's the same with Force Blast. It says "-40% Enemy Damage," but in the past, it's always been -x% Phys damage. So I'd like some clarification on the "-x% Enemy Damage" thing, too.
I played sorc recently and used both force blast and miasma to reduce enemy damage, I think it did help vs Assur in Duncraig(so spell damage, not just phys damage) but I'm not a 100% sure, maybe it was just placebo effect