What causes the black discoloration on this plastic electric teakettle? In the break room at work, there is a tea kettle that has some unappealing discoloration. I would like to use it, and lots of people do, but I find it terribly unappealing.

I can't remove it with soap, and my fingernail has no effect. I can't even feel it, so it doesn't seem like buildup. What causes this, does it adversely affect health or water taste, and is there a way to remove it or improve this teakettle?
See this previous post that explains how to clean an electric kettle. The suggestions include boiling citric acid, vinegar, or descaling solution in the teapot.

Presumably these are impurities from the water that have solidified on the plastic. It is possible that the lack of texture that you mention is because the impurities have bonded with the plastic.