I've never had them put anything to make it blue before. My LFS has put in Stress-Coat, O2, and the like, but nothing that makes it blue. Maybe some mix of Formalin and Malachite Green, I know that when they're in a mix, it gives a blue tinge to the water, but I'm partially color-blind. Anyway, if it is a mixture of these, they may be attempting to supress Ich, like when some LFSs keep their tanks at a low Salinity.