The easiest and most efficient way to get black burn off an iron is to use a mixture of baking soda and white vinegar. The acidic nature of the vinegar, the mildly abrasive nature of the baking soda, and the chemical reaction that takes place between these products will work hard to penetrate the scorching and allow you to wipe it away.
If you have a particularly stubborn black burn, however, you might want to leave the mixture of baking soda and vinegar to work its magic for around 30 minutes before wiping it off.