I sometimes add some cocoa, it depends if you want that flavor though. I have noticed if you color the dark colors. and let them sit a bit they darken up. so I think the trick is just that let it set a bit and take another look. Hope that helps.
I add cocoa to mine. Susan is right it does darken as it sits.
Take a look at this thread. I accidentally made black mmf with dark cocoa, if you scroll down a little for my post, it will tell you details!
Very, very carefully!! I haven't tried to the cocoa, that might work better than just using black coloring. I used black Americolors and it made the MMF REALLY hard to work with. It stretched and ripped all over the place. I said I'd never make black again.
Sorry for the bad news, but good luck!
I've seen colored fondant at Michaels. I think it was a box of browns and blacks. Might be the easiest solution! HAHA!
I made black mmf a couple weeks ago (used it for the wheels of my vw bus and black cat cookies), and I did add a little cocoa. I added before the powd. sugar along with the black color. It didn't take a lot of cocoa...When you taste it, you can taste a hint of cocoa, but not straight chocolate. The black color looks weird at first, but as long as it looks dark, just wrap it up and it will be black after it sits. 
merissa-what thread were you talking about? It didn't post
Duh, I forgot to post it! Sorry!
http://cakecentral.com/cake-decorating-ftopict-34064-mmf.html+turn+black
Hi ! I make chocolate MMF all the time and have never had a problem with it. If you check out my photos, all the black fondant was MMF. I use the large batch (like you said, with the 16oz bag marshamallows and 2 Lb bag sugar - but I rarely use a whole bag of sugar per batch). While the marshmallows are in their soupy stage, I stir in a melted square of unsweetened chocolate. Once that turns the whole batch brown, I add in the black color until it gets the shade I want - which is a whole lot less color than you use if you're coloring black from white. Then I add the sugar and voila! It works perfectly every time! And not using so much color keeps the fondant from getting sticky or stringy.
I've read of people using the cocoa with mixed results, but I've never tried it with fondant so I can't speak from experience.
Hope that helps!
Thanks merissa for posting that link. One question for you. What size is a "small batch" of mmf? The recipe I have calls for 16 oz. bag of marshmallows and 2 lb. of p. sugar.
I believe it is 1 cup MM
1TBS water
1 1/2-1 3/4 cup powdered sugar. As always it isnt an exact science! Sometimes I just make a couple of small batches intead of the big one.
You might have to do it by hand as well since it is so small for the KA
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