Sticker Stain

Bob LaCivita replies: Sticker stain is the result of acids and minerals leaching out of wood sticks (stickers) placed between layers of drying wood before drying or while drying (air dried).

You have a stain from metal. Depending on how deep it penetrated. You can sand, scrape or plane it out. If the board is not to thickness, you can run it through a planer. I would avoid bleaching because you might end up with a bigger problem. I would sand it with 220.