O earth, cover not thou my blood! There was a widespread belief in the ancient world that innocent blood, spilt upon the ground, cried to God for vengeance, and remained a dark blot upon the earth till it was avenged, or until it was covered up.
Job apostrophizes the earth, and be-seethes it not to cover up his blood when he dies, as he expects to do, shortly. And let my cry have no place; i.e. let it have no hiding-place, but fill earth and heaven.
Let it continue to be heard until it is answered.