Sorry for the late reply.
Maybe there is some kind of misunderstanding what exactly is the problem. When your projects reference assemblies from local project folder, you can't see our controls in Assets Tab or can't edit the control properties and design time throw error for some of the already added to the xaml controls.
I tried the following:
1. Created simple project and referenced Controls and Input DLL's from my desktop where I copied it from the installation.
2. Created RadCombobox and RadCalendar in the project and this was all made in VS2010.
3. Opened the project in Blend 4.0 and the Calendar and Combo where there and I was able to access their properties and edit them. Also in the Assets tab I can see all controls from Controls and Input dll's. They were in Assets->Locations->"Name of the dll" .
Could you please tell me if you are doing something else and what exactly is the problem. Any error you are getting after opening the project in blend would be of help. By the way are you putting the design assemblies directly in the folder of the Control's DLL's or keep them in a design folder.
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