Found this in the comments section:
A green-black stool? Black could be blood, green may be blood or bile or the color of something we ate. In any case, it's a great idea to have it checked out unless it's a very occasional finding. The power of pet poop (yes, really) | petMD
Since it's not normal color for poop, I would give the vet a call & see what they say, hope your guys will be ok!