Has anyone tried to do this? instead of on white buttercream i mean. If so can you give me any suggestions?

I haven't, but I don't see why it wouldn't work. Just do your design as normal and then for the background, use the chocolate BC just like you would the regular BC.

My myths of trivia cake was done on chocolate. It doesn't look the best, but it would have if I would've used my own frosting instead of canned. I was lazy and the cake wasn't for anything special so I ended up mixing canned with some of mine that I had leftover. My eagle cross country birthday cake was done with cinnamon buttercream. I would imagine the effect would be similar. I think you could pretty much do it on anything really! But then, I always fill in the entire picture (the size of the top of the cake) so that it's completely flat. Does that make sense? I don't know what's coming out of my mouth (or hands!) most of the time...forgive me if it doesn't make any sense!