Bests & Worsts Reviews from Amazon

Got a half sheet, rinsed it off and wiped it dry. We used it to make cookies. I added a little oil to the sheet and the cookies turned out great. afterwards, I wiped three sheet to get the grease off and noticed the papertowel was gray dark. I don't know what this is, or if you are supposed to prepare this sheet before using, but I really don't like three idea of some metal or whatever the gray stuff is on my food. It's this safe? Is it supposed to be like this?