When eating too many blueberries your poop should turn mostly black but your poop can turn a bit green along with the black or blue color from too many blueberries.
If your poop stays green then it could be caused by bile getting into your intestines and you should see a doctor if the green poop doesn't go away within a few days after eating the blueberries.
Other things that can turn your poop green are green foods such as broccoli and spinach because those vegetables contain a green pigment in large amounts called chlorophyll.