Why does sodium citrate in water turn black when blueberries are added?

I recently started making [oral rehydration therapy](https://en.wikipedia.org/wiki/Oral_rehydration_therapy) for someone I'm taking care of who developed POTS after COVID-19 infection. (ORT was recommended by their doctor.) I use the standard ingredients of ORT: sodium chloride, sodium citrate, potassium chloride and dextrose/glucose. To add some flavor, I tried blending in some frozen blueberries, but when I did the solution instantly turned black. The water became cloudy and opaque as well. By trying to recreate the reaction methodically, making four different solutions of blueberries and one of the other chemicals in water, I determined that this was a reaction between the sodium citrate and something in the blueberries.


Despite its off-putting color, the solution still tastes fine. However, I'm hesitant to serve it to the person I'm caring for without knowing for sure if it's safe to drink! What causes this reaction, and is it safe?


