Black gel food coloring is a MUST in my kitchen! It is so hard to achieve a true black color without adding a gallon of food coloring--and even then, you often end up with a dull gray instead. This Americolor gel is the best I've found--it's very intense, and will produce a deep black without the need to add a ton of additional coloring.
Pro tip: whenever possible, when trying to achieve a black color, I like to start with chocolate desserts. It's much easier to get to black when starting from brown, versus starting from white! So if chocolate frosting or chocolate cake works with the dessert you have in mind, I suggest starting with chocolate and adding black food coloring to that. You'll thank me for it!