What is the meaning of 'Rajasthan shades off into the fertile land of Gujarat'

Much of Rajasthan is the Thar desert and extremely arid. If you look at an aerial view, you will see that the transition to Gujarat in the south is gradual, in contrast to the transition to the Indus valley in the west, which is sudden.

If you were painting or drawing the gradual transition, you might do so with a shading technique with a succession of deeper greens, and that is metaphorically applied here for the physical appearance of the change.