Hopefully someone will be able to share their experiences with you on what this could be specifically but I have this link bookmarked about what the different stool colors might mean.
What Canine Stools Tell You About Diarrhea | beingstray.com
I'd still take him into the vet as soon as possible to try to better pinpoint what's going on, especially if the dark loose stools persist. Please keep us updated. I hope that it is not anything serious and that Charlie is back to normal soon!