Heaven Cakes!

Hi,

I have an aluminium baking tray that I use all the time. It looks
clean and shinny. When I try to greese before baking, I pour a little
bit of oil on the tray and use paper towel to wipe and even out the
oil. The paper comes out black, like pencil kind of black. What's
that black thing -- as I mentioned, the tray looks clean and shinny.
Will the black thing be a health hazard? Will the same thing happen to tin or stainless steel tray?

Thanks,
Banquo.