Black/ dark green-blackish stools on 2 month old

I know most of us aren’t doctors here but I would love to hear some advice from anyone who might have experienced this. My two month old was born at 37 weeks, he came a bit early. He’s been fine but for the past week or so has been having really dark stools and more diarrhea like. They started to get loose at the same time as his stool got darker. We have taken him to the ER but his labs and X-rays appear normal, but the stool issue hasn’t resolved. He does get fussy at times but he’s usually ok. When we go to the drs he’s most of the time cooing and smiling so the drs say that since he isn’t in pain it’s nothing. I’m glad he’s not in pain and appears to be fine but I don’t want it to go unnoticed either. Has anyone had their baby go through this? My baby is formula fed since birth. I did a little breastfeeding but then I went dry since I never produced more than 3 ounces. We try to keep him in the same formula style and brand if we are lucky to find which thank god we have been. He will normally just drink a different kind for a day. But I don’t know how to resolve it or what it might indicate.