MB was used as a textile dye originally, the improper disposal into water sources from industrial applications in is what this paper is referring to. Also from this paper

“Uses and Applications of Methylene Blue

MB is an attractive molecule with various properties useful for biomedical applications and is used as an effective therapeutic agent to treat anaemia, malaria, and Barrett’s oesophagus [72]. MB has primarily been used in human and veterinary medicine for several diagnostic and therapeutic procedures [73].”