Needed black fondant for a pink with black zebra striped cake, and my black fondant fell apart!!
I Use a homemade MMF buttercream recipe that I found here on the site. But this is the very first time I had to make so much black for one cake. It became crumbly and wouldn't hold together, let alone roll out. Needless to say I switched to hand painted black stripes!!
Mix a small amount of crisco with it. Black or any other intense color can be temperarmental. You might also try microwaving it for 10 seconds or so for one recipe.
I buy black for that reason. Kneading in some white might help, but it's really difficult to get those dark colors to not sieze up and crumble.
black is my favorite color, so i generally end up needing it on alot of cakes. if its more than just a small accent needing black, I airbrush it. HTH 
Oh, one other thing I forgot about that I do with my black. They make a black candy powder. I use that when I have to make black so I do not have to use so much of the gels BUT when you use these you have to let it set up a day and reknead it so the powder is mixed in well. You do not want to mix it in the day you need it.
I haven't used the MMF recipe with black, but have made Michele Foster's fondant black. I typically make it by reducing the amount of powdered sugar and substitute it for dark cocoa (dutch processed or Hershey's special dark). Then you've got a dark brown color and you don't have to use as much black food coloring. I also recommend investing in an airbrush. It will save you a lot of problems when doing dark colors.
I usually buy mine already colored, but I have painted white fondant black with food coloring thinned with vodka. (((Any excuse to pull out the vodka while caking!))) 
a little glycerine , in with strong colours can help soften it and bring it back to life.
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