There are a few reasons why your butter might turn black. If stored in a warm place, the fats can start to go rancid. If you store your butter on the countertop instead of in the fridge, this can happen. If your butter is starting to turn black, it's best to throw it out and start fresh. Another reason butter might turn black is because of a reaction with the metal. If you store your butter in a metal container, the fat can start to react with the metal. This reaction can cause the butter to turn black and taste bitter. If you notice this happening, it's best to switch to a different type of container.