How to make Black Marshmellow Fondant?

Replace some of the powdered sugar with cocoa to get a brown colour, then add Wilton black gel dye to it and work it in - it will take a lot. If you don't add cocoa first, you won't get it as dark without using TONS, and Wilton's black is actually a very dark blue, so the brown mixed with it makes it black. You could also try the cocoa, black dye and the darkest green you could find, the two dyes with the brown should work.