Pure copper(II) sulfate is white. It is also known as anhydrous copper(II) sulfate because it has no water in it.
When water is present in a sample of copper(II) sulfate it turns blue. It is still a dry solid, because the individual water molecules are trapped within the ionic lattice surrounding the copper(II) ions. Solutions of copper(II) sulfate are also blue.
This colour change can be used to detect the presence of water (or water vapour).