This is one of those quirks that until you try it, you may not know it exists or what the answer is.
I am working on a WPF project and have a separate control library (actually, a module because I am using the Component Application Guidance/PRISM pattern). I pulled it into Expression Blend and was surprised to see that I had no "design" view. The option simply didn't exist/was grayed out in the menu.
So, I did some digging around and it turns out that because my controls file is just a C# class library, Expression doesn't know it is "allowed" to design the controls. To tell it this, you simply need to open your .csproj file that the XAML is in, then add this to the first PropertyGroup tag that you find:
<PropertGroup>
...
<ProjectTypeGuids>{60dc8134-eba5-43b8-bcc9-bb4bc16c2548};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>
...
</PropertyGroup>
Just enter it exactly with the same Guids ... now close out of Expression, reload, and voila! you can now design.
