The toon shader shadow is always black. I have saw some people complainig about it too. I would suggest to do like the toon shader shiningness interface. It would be a bar for the shadow power/intensity. setting the bar to a low value would make the shadow more weaker,until it became the color of the material, and setting it with a high value would incress the shadow power, making it darker, until it became black.

I and others really needed this feature in ours toon projects. I only want a better tool for me and for all.