black residue in tea kettle
The black that originally came off is most likely mineral deposits. Those can easily be removed with vinegar or lime remover. 

The second part is a bit concerning and I would throw the kettle away since I don't think this is true enamel but possibly just paint. Enamel is very tough (think your teeth) and is hard to destroy. If you are able to scrub it off that is not a good sign. Then the brown underneath is problem rust from the metal. I would buy a new pot that is completely stainless steel.