LOL! What about using black food coloring paste. Just spread it on your tongue with a popsicle stick or spatula, let it sit for a minute or two and remove. Watch out though...if you get any on your teeth, they'll be temporarily black too, which is one of the reasons I recommend the paste rather than the liquid. That, and it's more concentrated.
for the Grudge last year, I mixed blue, red and yellow food coloring in a bit of water and just swashed it around in my mouth for a min or two. I had to repeat it after about 2 hours but the effect was great and totally freaked people out when I opened my mouth to make the Grudge gurgle sound.
One evening when you know you're staying in, test stuff out and time how long they last. I personally think food coloring is you best bet because any food/candy you do it with is only giving you a reaction because of the .... wait for it .... FOOD COLORING in it! LOL
Really simple. Chew on a couple of pepto bismal tablets. It turns my tongue black everytime! Even says on the package that it may turn your tongue black. Will also allow you to eat spicy things all night without a problem lol
If you don't mind the calories, there is icing in a can that is black, and it is yummy. It's kind of like easy cheese, but it is icing. It wears off after a little while, but you can just eat more of the frosting. I was a black demony thing a few years back and after much testing, this seemed to cause the blackest tongue.
Pepto tablets, (not bottle) are also pretty effective.
Tooth black also was necessary for my costume, but I'm not really sure what you are up to .