What happened when the cherry juice turned blue?
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.