How did they get this look?

It could also be ganache. You would apply a crumb coat, leave to firm up, then another thicker coat ensuring you make the marks in it with a spatula. With ganache the more you work it the more it tends to lighten up. With BC you might want to use a couple of different shades and blend them together.

The "official" style would be rustic, the "non-official" style would be monkey iced, as per Costumeczar
 

I agree it looks to be ganache to me . Love the dark chocolate brown coloured cakes.

So, I would make ganache and put it on, whip it and spatula it on to get the color difference and the swirl look?

that is so pretty!

it could be also IMBC colored dark brown. with IMBC, if you add more frosting to a cold cake, it tends to bring out different levels of color, but I would go with the ganache. nothing better than that!