Guys, there's totally green or blue food coloring in that bun.
Allow us to explain.
It's no surprise that eating different foods can change the color and consistency of your stool. Exhibit A:
And it's possible for an excess of green vegetables to give your poo a green hue. But we've seen the photos, and this ain't no kale salad situation. The most likely explanation is always the simplest one, and we're putting our money on green dye. The original Japanese black burger uses bamboo charcoal for both flavor and coloring.
But instead of burnt bamboo, the only ingredient of the bun that Burger King has mentioned publicly is A1 steak sauce -- and you better believe the super-black pigmentation doesn't come from there. It would definitely take some tint tweaking to reach the desired effect, and a dark green seems as likely a dye color as any.
Furthermore, purple or blue food coloring -- another, perhaps even more likely dye choice for a deep black bun -- has also been known to cause bright green stools. Ask not why the purple dye turns poops green, just go drink a bunch of grape Kool-Aid and see for yourself.
But since we don't have the ingredient list on hand, let's review other possibilities: Green poop can also be caused by the presence of bile, which can show up if bowel movements move a little more quickly than usual through the intestinal tract. This is nothing to worry about, health-wise, and it's probably not the first time fast food has zipped right through you.
Iron supplements are also known to cause a green tint in stools on occasion, but that would be a lot of iron for a hamburger bun.
We're sticking with our dye theory. The dyes have it.