The blue color of the hot water is thought to be due to the copper rust used in the bath kettle or piping.
Since this product contains citric acid, if there is copper rust (patina), it will be dissolved by citric acid and become blue copper ions.
As citric acid is also used for cleaning, it is thought that the piping part was cleaned with citric acid, but regarding copper blue water, the homepage of the Japan Copper Center (
http://www.jcda.or.jp/qa/tabid/95/Default.aspx) Is also described, so I will introduce this as a reference as well.
Copper is an essential element and does not affect the human body unless it is ingested in large amounts.
The main components of this bath salt are calcium, magnesium, carbonic acid, and citric acid. The hot water in which the bath salt is dissolved becomes a weak acid with a pH of about 5, and does not damage the bath kettle or piping. Please contact the manufacturer.