I've recently been searching for good oil blotting sheets, and the most commonly found cheap ones are Clean & Clear. I noticed on the ingredients list that it has mineral oil in it.
Is it possible that the oil in the sheets is supposed to keep your skin moisturized so it doesn't dry out and produce more oil?
I'm not sure, but I think they may impregnate the blotting tissue with the mineral oil to coat the pores you have blotted and prevent oil from sneaking back out of them. A shield of sorts. It would also repel water and sweat.
I love Clean & Clear blotting sheets. I have such oily skin and they really do the trick for me. I use them about once an hour. At this point, it doesn't matter to me what is in them because they work so well!