I agree with Aoper.
I'm not sure if the charcoal is toxic or not, but it's definitely not good. Even though your cat is scared, I would suggest putting it in the shower and gently (!) using lukewarm water, clean your cat. Whatever you do, do NOT use human shampoo or anything of the sorts on your cat as this could damage their skin and/or fur. You could however, use special cat shampoo. I'm pretty sure just some lukewarm water will do though. Your cat probably won't appreciate it, but I do think it's the best solution.
My cat fell into the dirtiest water once and I cleaned him up with some lukewarm water in the shower. He could've cleaned himself, but that would also have meant that my entire house would be covered in goo ;-) Turns out; he hated the slippery floor in the shower more than he hated the actual water.