M.B.
It is mostly the cooking residue, along with the non-stick spray. When you spray the can, even though it is mostly directed at the pan, it goes everywhere. (I've seen that residue on cooking equipment, also.)
Even though you don't fry, any cooking releases steam, which may carry small particles of oil, broth, and bits of food you can't see.
That dries on the blinds.
What about something like a 409 or Fantastik type spray? That is also designed to cut the oils.
I'm having problems with the pan inside my toaster oven... no amount of scrubbing will break down that oil residue that hadn't gotten cleaned up properly. (I rarely use it, but hubby and son do.... and of course, they don't clean it!)