A friend requested a cake like this in buttercream. How do I smooth the black and white without blending them. I'm afraid that if I use a spatula, I won't be able to keep the colors separate. Also, would it be better to put the white on first?

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

Jen, That's kind of what I was thinking, I just wasn't sure how it would work. Thanks for all the tips. Is your Dark Vader cake done that way?

Sugarfiend - I thought about doing a FBCT, but I don't think I have the freezer space for something that large.

The best way to make it smooth is to stencil it.

Rylan - How do I make a stencil for it?

Get some acetate and cut out the design with an exacto knife. You can also use those plastic folders. Maybe some type of thick paper would work.