What's the black stuff in my teapot?

Don't worry its just the Tannins from the tea. Without the development of the tannins, the tea would lack colour and its characteristic flavour. They build up over time and make the surface of the pot go black. It will also do the same thing to the inside of cups. Tannins are also used as a type of natural dye. Fabric can be soaked in cold tea to give it a light brown colour. To remove the dark tannin stain from your pot just fill it with boiling water from your kettle and pour some white vinegar in it, then leave it sit for a while. It will make it disappear. You may have to do it twice depending on how much build up is on the pot. I wouldn't use bleach or anything like that as bleach is chemical based and will alter the taste of your tea. Dont worry vinegar will do the trick and its safe and cheap :)