I made a stupid mistake
 

I've had my 2010 MINI Cooper for a week, and we are still getting acquainted. I was using single-edge razor blade to clean an old inspection sticker off the inside of the windshield, and the blade fell into the gap between the dash and the windshield, which I hadn't even been aware was there.
i think it could have landed on the cabin air filter, but I don't know. Now what do I do?

blade in the dash
 

First of all the cabin filter is under the glove compartment on the passenger side of your MINI. I would try using a magnet - run it back and forth where thr blade disappeared. Next you could try sliding a thin piece of card board in the crevice to see if it will lift the edge of the blade up so you could get it out. Try folding the cardboard until it is snug in the crack. (index card?) I would have a piece of duct tape rolled up sticky side out press against the blade to "snag " it. Also a # 2 pencil with a good eraser might make a good tool. Use the eraser end to push the blade against the glass sliding it up until you can grab the blade. Good luck! Be creative!

+1 on the magnet , it would not get to the cabin filter, that's on the other side of the dash.

It will be stuck on the joint between the dash pad and the metal seam for the windshield near the windshield black caulk seal.