I have tried to reproduce this issue on my side with our latest assemblies but did not succeed. Could you please make sure that you have correctly referenced the needed assemblies for using RadDataBoundListBox (Telerik.Windows.Controls.Primitives and Telerik.Windows.Core) in your Expression Blend project and restarting the tool? In case the issue persists, it will be great if you manage to send me your Expression Blend project so that I can check it out on my side. You will have to open a new support ticket to do so.
Thanks for your time.
the Telerik team
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