I have many of these exact tubes and all get black spots except this one, it is instead brownish. Does anyone know why?

Also, on another note, I have broken many T8s, and many of them have very rusted shields (even the ones with few hours on them). How can they be so rusted in a vacuum seal?

It is not the "rust" as a form of unwanted corrosion, but it is an intentional surface coating designed to help those structures work.