In Yellowstone, blue water is hot water. It’s blue because the water is too hot for bacteria to grow. This means the travertine lining the pool stays white, which makes the water look blue. The water looks blue because of all the colors that make up light, blue has the shortest wavelength. The reds, yellows, and oranges are absorbed by the water first. The blue wavelength gets scattered and makes the water appear blue.
I learned this from my Grandpa during my first trip to Yellowstone in 1986. Grandpa was a chemistry teacher. I suspect I learned more about the natural world from him than from almost all the other sources combined.
The other nice thing about hot water in Yellowstone is that the steam rising from it makes for some great compositions, like this standing dead tree at Canary Spring.