Here's why:

* Anhydrous copper sulfate (CuSO₄) is a white powder.

* When it reacts with water (H₂O), it forms hydrated copper sulfate (CuSO₄·5H₂O).

* This hydration process is exothermic (releases heat) and causes the copper sulfate to change color from white to blue.

The blue color is due to the presence of water molecules coordinated around the copper ions.