Shortening would make it more moist. If you have access to black fondant, you can mix it 50/50 with the modeling chocolate and that will take care of any elasticity issues, as well as help it hold its shape.
Does the Merckens black chocolate have a funny taste? The ones I ordered for Halloween tasted funny. I wasn't sure if they were old or if the black just gives them a funky taste. I thought they were awful.