It started with a Parisian hairdresser in the 1920s who liked to make a big entrance. His blue convertible was sparkling but he thought his Russian wolfhound’s coat looked dull, so a pharmacist told him to add a few drops of bluing (a laundry solution used to brighten dingy whites) to the rinse water the next time he bathed the dog.
Actually, gray- and white-haired women were already adding a drop of bluing or indigo to a shampoo’s final rinse. Commercially made blue rinses became available in the 1930s. One was supposed to choose the appropriate tint based on hair color (iron gray, silver, white, etc.), but it was still easy to overdo it. “The solution was too strong. The blue stayed,” explained a woman whose appearance on a Chapel Hill, NC, street generated a 1933 news story.
[from an article in Bust magazine]