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Blueberries are packed with anthocyanins, which are the pigments that give them that deep, royal blue hue. These pigments are powerful antioxidants, but they’re also little troublemakers when it comes to color.

Your body doesn’t always break down 100% of those pigments, especially if you eat a bunch at once. So, what doesn’t get absorbed in your small intestine heads south to your colon, where it mixes with bile and other waste.

The result? A lovely shade of dark greenish-black or even jet black. It’s like you’re a human tie-dye project, but only on the inside. (Please don’t try to sell the pattern on Etsy.)

When to Actually Worry (And When to Just Giggle)

Here’s the golden rule: if you’ve been eating blueberries (or black licorice, or taking iron supplements), a black stool is usually just a high-five from your diet. It’s like your poop is saying, “Hey, you really liked those blueberries, huh?”

But please, don’t ignore other signs. If your black stool looks like tar—sticky, stinky, and thick—or if you have stomach pain, dizziness, or a weird metallic taste, that’s a different story. That could be a sign of internal bleeding, and you need to call your doctor immediately.

The easy test? Try the “smell test.” Blueberry-poop usually smells, well, like normal poop but with a faintly sweet or fruity undertone. Bleeding-poop smells… worse. Like a rusty, sickly sweet smell. If your nose raises an eyebrow, trust your nose.