I don't have a recipe for roll out (it's much easier to just buy it). But I do have some tips I wanted to keep track of.
To get a good black colour, get some cocoa powder and mix it with a little water to make it a paste, then knead it into the icing before adding your black gel paste colouring. Careful though because this can make the icing too sticky. You might need to dry it out in the fridge for a while.
I've also heard you can get a good black by mixing green, violet and black. But I haven't tried this yet.
If you want to make it shiny / glossy or wet looking, combine 1 part vodka with 1 part light corn syrup. Paint it on and then when dry it will be nice and shiny. Avoid touching it though as it will leave fingerprints.