I would do the cake black and smooth then pipe on the white in star tip or something similar. Nothing to blend.
Is this something that could be done as a FBCT?
I used to just use the star tip for all of my fill-in work. However, I have discovered that I really like the smooth look much better-it just looks a little less amateur to me.
The way I would do that cake is this: Ice and smooth the whole thing in black, then use a toothpick or something similar to outline your design on the cake. With white icing and a smallish round tip, maybe size 4 or 5, go over your outline and then fill in the entire area using the same tip. Before it crusts, use a small spatula to lightly go over only the white area, just to smooth the lines that you made while filling in. After it crusts a little, use a Viva paper towel to smooth the white area completely. Lastly, use a tip #3 to outline the entire white area one more time, this will "clean up" the edges. Then add any final details.
I have a couple of pictures in my photos that use this method and it always works well for me.
HTH 
Quote by @%username% on %date%
%body%