If you don't have one I've heard you can get the spray stuff at Hobby Lobby.
I cheat....I buy a can of Pillsbury frosting, put it in a large bowl. I add Wilton black to it until it looks dark black. Then I start adding powdered sugar until I get it the consistency I want. When I took the cake decorating classes, that is what my instuctor did....and she owned a bakery in NYC for 20 years.
It is easier to get black by first starting with brown icing (either colored brown or chocolate) and then adding black color.
I agree with raquel1, if it is for a large part of the cake, use an airbrush or the edible sprays.
I personally start with dark chocolate icing, and add the black coloring from there. I have found that the more gel coloring you use the funnier the taste of the icing. And of course, people don't end up with black teeth.
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