Phil.....that blue coloring usually comes from drying blue jeans washed in cold water....
most times if its light enough, with normal use it will return to white.....but this does involve washing in warm/hot water from now on......
I have seen some so dark, there is no return to the original color....a lot of times this blue in the dryer tells you a lot about the washing habits of the owner, not to mention the condition you would find the washer in.....