Each time that I have made Black Frosting I did it freehand, so to speak. I never had a recipe to follow. Now days You can buy black food coloring, but I find it more fun to mix the color myself. I start with a container of white frosting in either vanilla or cream cheese flavor. If you have a box of primary colors(red, blue, green, yellow) food coloring it will tell you how to mix purple. This is a good starting point. Once you have the basic purple you can add small amounts of green. Always check the flavor of the frosting while mixing because sometimes too much food coloring can make it taste like dye. Personal preference comes into play now. You can make it a grayish black or a charcoal black. If you don't care about the flavor you can keep adding food color until you get a darker black.
The first time I ever made black frosting I used red and green. I used mostly green to get it dark and then added red, which turns it a sort of brown shade. From there I added more green and red drops in small amounts until it was a dark color that resembled black. In the end it all depends on how you want it to look.
I have never tried to make black frosting using the already green frosting but I suppose it could work because I have used orange halloween frosting to make a salmon-red color.
If anybody does try to make black frosting please send me a picture so i can see what it looks like. I haven't ever tried to tell someone how to do it before. I hope it works. Happy Coloring!