Please see my comment to the question. I hope you know all that, but just in case: this article provides sufficient explanation on how MVC should work: http://en.wikipedia.org/wiki/Model-view-controller
See also http://c2.com/cgi/wiki?ModelViewController
From the first attempt of Google search, I could not see any useful material on using MVC with Expression Blend.
I tried out Expression Blend, and, frankly do not see this product as a serious or comprehensive software development tool. In my impression
, this is rather a fast prototyping tool with a strong focus on the aspect of design in the sense of "industrial design" and "UI design". Perhaps MVC or similar architecture can be developed only on the stage of migration of the prototype into a fully-fledged feature-complete product. I don't want to insist on my point of view as it can be merely a product of my insufficient experience with this software.