I know this is late to the game, but I wanted to add that I needed some black choc clay so I kneaded in some black candy color into brown clay. I needed 3 containers of the oil to finally get the clay to turn a very dark brown (but not actually black). Having added so much coloring completely changed the texture of the clay and I could only roll it out a couple of inches before it started cracking. Also, I couldn't roll it out even remotely thin. I figured the problem was the clay had been overworked from stirring so much.
The next time I needed black clay I decided to add the color at the point of melting the choc (before stirring in the corn syrup). It still needed a lot of color to get dark (again, never turning black), but I didn't have to work it as much so I thought I had solved the issue. Not so. When it was time to use it the clay was also grainy and hard to roll out. Much less so than the first batch I made, but definitely not as easy to work with as all the other clays I make and work with.
Since I am most definitely not a chocolatier I figured it was my lack of knowledge so I stopped in to my local custom chocolate shop and talked to the owner and he told me that it's impossible to male black choc (not clay, just chocolate) because the color turns purple overnight from fading. He said the best he can get is a really dark brown.
I have no idea if what he told me is correct, but from my personal experience, black is a really hard color to achieve and once you get the color the end product is really hard to work with.
I'd love to hear if others have a better method in case I ever need black again because I've never seen black Wilton melts at my local store.