What happened when the cherry juice turned blue? When I was eating cherries today, some of its juice spilled on my shirt. Attempting to clean it, I poured a solution (baking soda, vinegar, toothpaste, and salt) onto the fabric. Though it did not remove the stain, the colour turned from a wine red to a subtle blue.

So the Question is: Why did it happen?

(This is not that technical so I'm not sure how to classify it)
Cherries, like red cabbage, hibiscus and many red or purple colored fruits, contain a class of compounds called anthocyanins of general formula:



where $\ce{R_x}$ is either $\ce{OH}$, $\ce{OR}$ or $\ce{H}$.
In basic medium it loses a proton and it converts to a non charged chinone like structure, which has a blue color.