If there are flowers, that will cover your seam of colors. It doesn't have to be perfect, then, but I'd make it as neat as possible. Sort of 'draw' your line for color separation, and go for it. I'm sure you'll do fine, it's just something new.
I was thinking that as well...if there are flowers on the backside to cover the seam there, then I would mark the cake where the seam will be, apply the black buttercream up to the seam, let it crust or harden (depending on what type of buttercream you're using), then putting something (a benchscraper? ruler?) where the seam is, do the white side.
The flowers will hide the seams.
Having said that, I've never done one, so this is only in my head. 
Start with chocolate buttercream, then add your black colour paste. Trying to get black from white buttercream is almost impossible without messing up the buttercream.
The cake in your photo looks like it was done in fondant, which you can make shiny through various methods, but I don't know how you can make buttercream shiny. I don't think you can.
This just so happens to be my cake ! So cool.
I would recommend to your customer that this cake be done in fondant, or at the very least the black side. That way it can be peeled off and not eaten, that much food coloring will turn all the guests mouth a lovely shade of black! You can't peel off black buttercream.
There is a seam down the back of the cake, it was unavoidable, but I made it neat and straight and piped a pearl boarder down it so it wasn't all nakey.
.
This just so happens to be my cake ! So cool.
I would recommend to your customer that this cake be done in fondant, or at the very least the black side. That way it can be peeled off and not eaten, that much food coloring will turn all the guests mouth a lovely shade of black! You can't peel off black buttercream.
There is a seam down the back of the cake, it was unavoidable, but I made it neat and straight and piped a pearl boarder down it so it wasn't all nakey.
.
pieceofcaketx, I'm copyrighting the half and half cake, so I insist that you stop making this design immediately and pay me back royalties! Hahaha! Seriously, I have one this week where I have to do the half white half chocolate version of this,and I was just going to cover the seam on both sides with flowers. Mine will be buttercream, though, so that eliminates the fondant issue.
if you start with black cocoa powder you're halfway there in more ways than one ;)
don't have to use as much black color
http://for-truly-dark-chocolate-baked-goods-black-cocoa-powder-ingredient-spotlight-173923
hmm--guess that's cutting it close for this saturday though!!!
duh on me-- maybe it's available somewhere locally--but an idea for next time anyway
(wow it's into march already)
Quote by @%username% on %date%
%body%