A 25-year-old woman turned up at a Rhode Island hospital with blood that had turned navy blue. According to a paper in the New England Journal of Medicine, the woman had used a topical pain reliever for a toothache. She was diagnosed with “acquired methemoglobinemia”.
Source: https://www.theguardian.com/us-news/2019/sep/19/woman-navy-blue-blood-rhode-island-hospital