Sorry, but building blocks *is* the replacement for the UI - there was a panel at CitCon and an entire episode of ISC on the topic last year.  

What may have confused you is that they also explicitly talked about the work they're doing to decouple the UI itself from the underlying functionality, and moving towards a proper 'Model-View-Controller' setup for the UI (which the previous legacy flash framework didn't support, and required the UI to be tightly coupled to the functionality - which was another pain-point)

In short, 'Building Blocks' is both a framework, and an improved architecture, and a set of UI components (in HTML) and stylesheets (in CSS).