hello, welcome to just answer. my name is XXXXX XXXXX and i will be happy to assist you with your problem. This is normal with all of the porcelain lined dryers. some people say it is caused from the dye of clothing but i tend to believe it is just from the heat on it over time as it is always the same color blue on every dryer. it will not hurt anything. this is on almost any dryer 5 years and older.
I have even used strong chemicals to try and remove this discoloring and had no luck.