Does + to Magic Damage on a weapon effect your spell damage?

The subject is slightly confusing to me. When I first started, I ran into a sword that gave +30 to Magic Damage. With it, my Magic Blast damage went from 20s to 50s. Then, I got a second +Magic Damage sword, and dual wielded them. Yet, I never noticed a second increase to Magic Blast.

Does it effect my actual spell damage, is it only working on one weapon, or what?