I think you will have better success with longer lasting playdough if you use just food coloring in place of kool-aid.... just my personal experience... even when using food coloring (or liquid watercolor, which you can buy
here, and is awesome) and some sort of extract as scent, I have better luck than kool-aid

the cream of tartar in your recipeis to help the playdough last longer with out refrigeration.
PP's are right... anise should give it some very nice licorice scent (sounds like a great idea!)
You can play around with using all the colors of food coloring to get black-- as you're doing this- mix the colors in the measuring cup you are going to use for the water, then when you get the color right add the water up to the 1C mark- that way you're not adding too much liquid when you add the food coloring and throw off the recipe.
I highly suggest ordering Liquid Water Color from Discount School Supply- you can order black, which would make your job super easy, and unless you're in Alaska/HI/ out of the country, shipping is usually unbelievably quick.
I prefer the water colors, because if I'm going for a vibrant color the food coloring can sometimes come off on LO's hands, and the watercolors don't.
If the kinds aren't super small, you might consider adding some silver glitter to your playdough- always fun to do, but especially with black playdough.