Funny, I tried something similar back when I was a hungry teenager. I could never get the peanut butter to blend. All it would do is clump to the bottom or side of the blender. When I would mash it down to the bottom where the blades are, it would either gum up the blades and bog the machine down, or the blades would just spin in the cavity of the peanut butter, without mixing it with anything else.

Surely there's some secret to getting peanut butter to blend (especially when it's cold, like when you're putting it in with ice cream), but I couldn't get it to work. Maybe I should try it again now that I'm grown and understand physics a little better.