Are you absolutely certain that the part facing the camera is the side of the mold? I have to say that it looks very much like the widest part, that one facing the camera, is the top of the mold and they turned this long slab bar on its side for the picture. I haven't run across any molds that are one bar wide and deep, where the short side of the bar is the top of the mold.
At any rate, it does look like they used about 50% black and 50% white and swirled it at a thin trace or before trace. You can mix the lye and oils with a whisk or stick blender until it's just mixed ( and stays mixed) but well before it has started to thicken up. THen you'd split the batch and add your colorants and mix with a whisk or spoon and then add your fragrances. Stir it in well, without using the stick blender. It should still be thin and shouldn't be at more than a thin trace (I wouldn't use a fragrance that accelerates trace for this). It looks like they poured in one color, than the other, and alternated. For swirling, you could use a straw and go all the way down to the bottom, or if you are certain that this was a higher than wide mold, you would have to look around for something you could shape into an L shape- a coat hanger, for instance. Plunge it down into the soap with the long side that you're holding right up against the side of the mold and pull it up and down and around throughout the depth of the mold, so the short side of the L is doing the swirling. It's tricky to get it emulsified enough not to separate but not stirred to trace (believe me, I've been working on it) but it can be done.
Post any pics when you've tried it! I would love to see how it works out.
