The RadControls for WPF can be fully customized using ExpressionBlend and we are not aware of any issues when displaying a UserControl/Window containing a RadTabControl
in Blend. I attached a sample project that was created in ExpressionBlend 4 and you should be able to open and view it on your side as well.
In your case the issue might be caused by different versions of the assemblies referenced in your project. Can you give us more details about that. Also, if you can open a support thread and send us a sample project reproducing the issue we will look into it and get back to you with a solution.
All the best,
the Telerik team
Explore the entire Telerik portfolio by downloading the Ultimate Collection trial package. Get now >>