The darkness element itself was a very versatile element that had a specialty of being versatile.
And if the darkness element was to be added to another element, it would strengthen the other element, giving it a superior quality compared to the original quality that the element had.
And in the case of adding it to an earth elemental spell, the darkness element would make the earth tougher, and give the earth construct an absorbing quality that made it even more resilient to attacks.
And that absorption was also the characteristic that had saved Blake from receiving internal damage to his body despite him receiving a monstrous punch from the skeletal wing demon back then.