Shadow is now an official element with the release of titans.This element is currently unlearnable. Shadow has only been shown by 2 features in the game so far: Pippet and Titan. Pippet uses one shadow attack in the battles against him, Shadow Shock. Shadow Shock is presumed to be the Shadow version of (element) blast. The other attack seen by the Titan is Royal Roar, which looks like Zero, except that the magic is dark purple.
By what we know so far, Shadow is the element opposite to Astral. Shadow is weak against Astral, but strong against all other types. Astral is the other way around, where it is strong against shadow, but normal against other elements. The question is, why is the titan a water element? Why not make more Shadow spells? They should even make shadow pets!