If you are using a crusting buttercream I'd just use black buttercream and place the shapes around the cake til your happy with how it looks and then smooth each spot with a Viva. Unless you are truly wanting to "paint" it on?
You could use paste or liquid food coloring thinned w/Vodka (or similar/everclear) but that could cause people eating it to have black tounges/mouth and if they drop any on their clothing it will stain. I think gigglebox is right on. Make your blk b'cream by starting out w/choco, then adding some paste color.
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