Outlet box mounted in rim joist
A rim joist isn't under load the way a floor joist, so the usual restrictions on notching and boring floor joists do not apply.  Rim joists are cut all the time for dryer vents etc.  Just make sure you're between floor joists / studs.  

To hedge a little, I would still prefer to mount the box horizontally and centered on the rim joist, just a bit stronger that way.  

I will assume there's no way to go up or down a few inches and clear the rim joist, or surface mount the box.  That would be much easier and I'd still prefer not to weaken the rim joist if possible.