Prepare white cake according to package directions. Take half the batter and and add 2-3 drops of black food coloring and mix to make a gray batter. Pour the gray and white batter into two cake pans and bake according to package directions. Cool completely and thencrumble into two bowls. Add 1-2 tablespoons of vanilla frosting and mix until crumbs hold together.
Repeat with the chocolate cake preparing according to box directions. For black rats, add a few drops of black food coloring to the batter, omit for brown rats. Pour batter into two cakepans and bake according to package directions. Cool completely and crumble only ONE of the cake layers into a medium bowl and add 1-2 T of chocolate frosting and mix until crumbs hold together. (Frost the other layer and eat as is. :)
Work with one color at a time, beginning with the white cake and press cake crumbs into rat molds and overfill the molds so that the rat has a little girth to it so that you'll have a big enough piece of cake to stick the candy cane tail into. Gently turn mold over, supporting the rat cake in your hand and bend the mold starting at the tail and the rat cake will easily fall into your hand.
Place molded rats on a wax paper lined cookie sheet. Line up molded rats, cover with plastic wrap and refrigerate for at least two hours or overnight to solidify the cake pops. While the cakes refrigerate, prep the blackberry sauce and reshape the candy canes into curled tails.