I have a lot of grad hat requests, but most want buttercream. Won't this turn your mouth purple?
I figure with fondant the purple mouth won't happen and would be much better....
I haven't had too much trouble if it's just used as a border. I the whole cake is black buttercream, then yes, I think you'd be better off airbrushing.
I made a black buttercream wedding cake (in my pics). I warned people before hand to be careful with it, but I was really surprised that it didn't cause as many problems as I thought. It wasn't even as bad tasting as I thought it would be, but of course I started off with chocolate buttercream so that might be why.