Blueberries usually make your poop turn a black color instead of dark green.
Most times a dark green or green colored poop usually happens due to eating foods such as lettuce and other green vegetables, such as spinach and broccoli, because these foods contain large amounts of the pigment chlorophyll which is green.
But sometimes I've had green poop as a result of eating too many blueberries and other times the poop was just black.
But the color of your poop can normally change from time to time depending on the types of foods you eat.
Even taking some medications that contain bismuth subsalicylate, such as Pepto-Bismol can result in black poop which is normal as well.
If you're eating blueberries and your poop turns black or green then it's a normal thing and nothing to worry about but if your poop stays green or too black and doesn't change back to normal brown then something could be wrong.
In that case you should see a doctor about it to get tested for some other problems.