Historically, indigo was a natural dye extracted from plants, and this process was important economically because blue dyes were once rare. Nearly all indigo dye produced today — several thousand tons each year — is synthetic.
The restraining device is apparently called a ‘scavenger’s daughter’ and was invented in the court of King Henry VIII. Rarely used historically, there is one genuine one known to exist that is still in the Tower of London. (xpost from /r/Bondage)
