I did the cake that toko posted a link to, for my sister's 25th birthday. I used mostly a size 6 tip, and the black icing was chocolate tinted black. I always liked piped things that look like they are piped, so I didn't smooth a lot down. My biggest mindset was to just be loose... a little curvy with not a lot of rough, geometric edges, and each stripe a little different.
Word of advice, at least with the cake I did... the black will bleed into the white, so don't do it too early. Fortunately, mine cake was for later that evening, so the bleeding didn't happen until it was leftovers.