I have just tested the behavior you described however everything works fine by my side. The Intellisense appeared and the corresponding theme is applied correctly. You may see the result at the attached pictures.
May you verify that such a behavior can be observed only in Microsoft Expression Blend?
Please try the following one and let me know what happens:
1. Create a brand new Silverlight project in Blend.
2. Manually add a reference to Telerik.Windows.Controls.dll and to Telerik.Windows.Themes.Expression_Dark.dll.
3. Drag a single RadButton onto the ArtBoard and apply this theme to the button.
Generally when you update RadControls using the VS Extensions, Expression Blend is not updated automatically. In such case you have to manually add the references you need in Blend environment.
We will improve this in the future and Blend will be updated automatically.
Hope this helps!
the Telerik team
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