Explain why copper 2 oxide is a base although it does not turn litmus paper to blue​

Copper (ll) oxide is a base, but it can't turn litmus paper blue because it doesn't dissociate to form H+ and OH- ions, but when it it dissolved in water then it get converted into ions and hence it changes colour of pH paper into blue.

Anhydrous acids or bases doesn't turn pH paper into red or blue because they don't form ions, when mixed with water they start showing their characteristics.