If you're using something like MVC, MVP, MVVM, etc, then the UI doesn't talk to the BL, but uses an intermediary

Yes, though those are tethered to/associated with the technology. The point (which most know) is that whatever layer the UI works with, it shouldn't work with the data layer.

(And it's amazing that relatively recent items like the SqlDataSource and TableAdapters almost encourage violations of this)