I'm sorry to disappoint you but right now you cannot use EditControlTemplate functionality in Blend to edit styles for RadControls for Silverlight 2. This is because we are using our implementation of ResourceDictionaries which is incompatible with Blend 2 SP1.
We will try to fix this issue for the coming Service Pack (around the end of January).
I'm sorry for the inconvenience.
the Telerik team
Check out Telerik Trainer
, the state of the art learning tool for Telerik products.