I do if I really need a dark black. If not, I'll try to just use food coloring, but it's really hard to get the right color without diluting your frosting. After all, some outlines are not too much, so it might be pretty subtle, and not affect your cake flavor that much, if at all.
You mean there's a flavor that doesn't "go" with chocolate?!?! 
I agree with the previous poster.
What I did to solve this problem was make a half batch of FBCT icing, and thin it with black coloring instead of with milk (well - I did have to add a little milk). I keep it in the freezer for whenever I might need it.
If you are making a small amount of black you can do it with out starting with the chocolate. I start with a thicker icing, than what constancy. After you add the color you can always thin it down more, but thinning seems easier than thickening it up.
Add the color until you get a dark grey, then cover it and walk away from it for a while. The smaller amount you do the faster it will darken all the way to black. I have had dark gray to go to black in about half an hour with about a 1/4th of cup of icing.
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