I was making some BC tonight and added more liquid than I should have for the amount of powdered sugar I had (thought I had more than I did). I REALLY do not want to go to the store tonight or tomorrow (need the cake tomorrow night). I've heard you can make powdered sugar (in the blender, maybe?). Has anyone done this? I've searched here and on the internet w/o much luck.

Or is there anything else I can add to make my medium-thin BC a stiff BC?

I have done that before. It doesnt get as fine but it works in a pinch.
Good luck

Do I need to add anythign to it? Cornstarch? Or just the powdered sugar blended until fine(r)?

Cornstarch content is around 3 to 5% in commercial icing sugars, just so you know should you decide to add it. I don't for certain know if that's by weight or volume, though I'd presume it's weight. Using the 5% amount, that would mean 3/4 of an ounce per pound (16 oz.) of sugar.

I know you can add cornstarch to your royal icing to thicken it up if you add too much water. You may be able to do that with your buttercream. You have to let it sit a while after you mix it up though.

Add a 1/8 to 1/4 flour to your buttercream. I have a recipe that calls for flour and it is a wonderful buttercream recipe. I would start with 1 tablespoon and go up as needed.
Judy