You've shaded the cracks the opposite of how they should be. They presently look as though they are protruding from the block. Swap the light and dark shades. Look again at ptoing's edit.
The blocks could really do with some subtle texturing of their surfaces.
Would look more believable if the blocks weren't all perfectly aligned. You could have some sunk a pixel or two deeper, or sitting on a slight tilt.
Maybe some sand drifts blown against the blocks could better seat them in their environment.