We've started work on a WPF application a client. They have an in-house designer that they would like to use for doing the visual design of the application. This I have no problem with. We just did something similar with a webapp we created with them, and it worked well.
We've pointed their designer at Blend, and he's going to go off and learn while we're doing development on the application.
We already have a shared Dropbox folder that these resources will probably live in, as our in-house source control is (understandably) locked down, and I doubt I'd be able to introduce something like a shared Mercurial repo.
What I would like to do is share the bare minimum with the designer, only the xaml files if I can. As this is my first large WPF project, and my first one involving Blend, and an outside designer, I'm not sure on the best practices for this, and would like to get input before moving down this path.
I use Blend quite a bit and for the most part the interface and an outline of the Model and CodeBehind/ViewModel is all you need.
I typically do all of the work, (Design and Code), but Blend can host design time data which can be created using a model. If I have a very well designed and documented ViewModel I can pretty well write an interface around it. If your Model and other logic are going to be in binaries that you send them, be sure the exposed members are well documented so that they know what to expect behavior-wise.
If you can get approval to release more of your code to them, so much the better. Actual code goes a long way above and beyond documentation.