Blend only works with parametric primitives of the same type, so you need to start with two cubes, not a cube and a sphere. Give them plenty of segments for a smooth deformation and add a spherify deformer to each, with a strength of 0% on the first one and 100% on the second. The first clone can be flattened, as yours appears to be, but the second needs to be of equal dimensions so that a full sphere is formed. The cloner will then blend from one to the other.