Why is our bathroom water leave a blue stain?

I this might be your problem. Hard water deposits can accumulate on the sides of the toilet bowl. Then if you clean the toilet using something like Vanish, which turns the water blue, it can cling to the mineral build-up on the bowl. Kohler does not recommend using Vanish. To get rid of these stains, you can try some muriatic acid (diluted of course.) You only need about a 10% solution. Be very careful so you don't burn your skin or eyes if it splashes; wear rubber gloves and eye protection. The brush you use with the muriatic acid should not have any metal that can come in contact with the acid in the toilet. There are brushes available that do not use heavy wire to hold the bristles. A good cleaner is Zep available from Home Depot. This is a good alternative to the muriatic acid.