It always says "The solution "foo.sln" is of an unsupported Format and may not open correctly. Do you want to open the solution anyway?" If I click yes, it Shows the solution tree, but when I open a xaml file, it only Displays its xaml text, not the Rendering of the UI.
According to this http://blendinsider.com/technical/available-now-blend-for-visual-studio-2012-2012-08-15/ Blend 4 is the way it's meant to work. So something is wrong I guess.
I have older Projects created with vs2010 and Blend 4. I can still open and work in them in Blend 4 while working on them in vs2012. Only new Projects created with vs2012 seem to have this Problem for me.
When you open a VS2012 project in blend 4, a message like this is showed:
The thing is that if you click the yes button then it will open the project, but if the target framework of the project is 4.5 then the blend only will show you the Xaml code, and no more else. But if the target framework of your project is 4.0 or lower, then you will may edit the project as normally you have done before.
Hope this could helps you...
来源：https://stackoverflow.com/questions/13518417/how-to-edit-a-c-sharp-project-in-blend-4-that-was-created-with-vs2012