I have a new theory; the pasta with the Oaxacan herb (not that herb smart-alecs) was black. Usually that’s from using cocoa or squid ink, but if they used food dye that would explain it
Purple food dye makes your poop green.
I’ve tested this with grape pop, various grape candies and other purple substances.
I assure you, this is the answer you seek.
I have pooped green turds after drinking green Gatorade or after eating wraps with the tortillas dyed green.