Interesting!
I tend to get very dark green stools from Perfect Food (super green supplement) and blueberries. Are you eating blueberries or blackberries? :)
Green stool usually happens when there's oxygen-friendly microbes helping to further break down the fecal matter. The bile salts/bilirubin usually attract microbes that oxygenate things. So green stools are often due to eating some kind of oil/fat.
Many CureZone members have reported seeing dark green, almost black, stools from taking a good intestinal cleanser, and some consider it a sign of success, as in the cleanse is almost over and done with. In those cases I think it's a combo of mucoid placque being released but also a cleaner bowel allowing more healthy microbe activity.
Be sure to take in a bit of butter or olive oil with your carrot juice! :) It will help the beta-carotene turn into Vit A.